Nagarjuna's MMK should not be taken as an argument against the existence of a first 'Cause'. As a Non-dualist he does assert that there is an Absolute Reality; he is not a nihilist. Prof. Chandradhar Sharma has an extensive discussion and analysis on this in his book A Critical Survey of Indian Philosophy. He says (pp 89-90):
Before the mighty strokes of the destructive dialectic of Nagarjuuna and his commentator Chandrakirti the entire structure of phenomenal objects crumbles down like a house of cards or a palace on sand. The external objects and the individual subject, matter, motion, causality, time, space, thinghood, qualities, relation, attributes, substance, soul, God, religion, morality, the four Noble Truths, Nirvana, and the Buddha are all hypostatised relations. But from the empirical viewpoint they are all quite real, though ultimately they are all merged in the bosom of the Absolute.
and further on pp 93-96:
The Shunyavadin is neither a thorough-going sceptic nor a cheap nihilist who doubts and denies the existence of everything for its own sake or who relishes in shouting he does not exist. His object is simply to show that all world-objects when taken to be ultimately real, will be found to be self-contradictory and relative and hence mere appearances. True, he indulges in condemning all phenomena to be like illusion, dream, mirage, sky-flower, son of a barren woman, magic, etc. etc. which suggest that they are something absolutely unreal. But this is not his real object. he indulges in such descriptions simply to emphasize the ultimate unreality of all phenomena. He emphatically asserts again and again that he is not a nihilist who advocates absolute negation, that he, on the other hand, maintains the empirical reality of all phenomena. He knows that absolute negation is impossible because it necessarily presupposes affirmation. he only denies the ultimate reality of both affirmation and negation. He condemns intellect from the ultimate standpoint only for he knows that its authority is unquestionable in the empirical world. He wants that we should rise above the categories and the contradictions of the intellect and embrace Reality. He asserts that it is the Real itself which appears. he maintains that Reality is immanent in appearances and yet it transcends them all, that Reality is the Non-dual Absolute, Blissful, and beyond intellect, where all plurality is merged. This is the constructive side of the of the dialectic in Shunyavada which we propose to consider now. Here intellect is transformed into Pure Experience.
...There are two standpoints--the empirical and the absolute. The former deals with the categories of the intellect (koti), with name and form (nama-rupa), with dependence (nimitta), with relativity (vikalpa or sanga), with practical compromises (nama-matra), with phenomena or appearances (vyavahara or samvrti); the latter transcends the former and deals with Perfect Knowledge (prajna-paramita) which is Non-dual (advaya), Independent (animitta), Real (sara) and Absolute (paramartha). Ultimately it is the Real which appears. The Real is at once immanent and transcendent. The suchness of all dharmas is the suchness of Reality. The phenomenal is the noumenal and the noumenal is the phenomenal. Appearances are Reality. They are grounded in the Real, the Brahman which at once transcends the duality of the relative and the absolute. They are not two reals set against each other. They are not diverse, they do not form a duality. It is only from the absolute standpoint that we realize the true nature of the world. But the phenomenal is not to be utterly condemned; intellect need not commit suicide because it is from the phenomenal that we can go to the noumenal, it is from the the lower that we can go to the higher. From the empirical viewpoint, the Buddha, the Bodhisattvas, Religion, Morality, Doctrine, Truth, Nirvana, nay all the dharmas do exist. We shall rise to the Absolute, not be denying the relative, but by transcending it with its own help.
...The Lankavatara also declares Reality to be a Spiritual Experience which is beyond the categories of the intellect, beyond discrimination (vikalpa) and dualism (dvaita), and which can be directly realized by the Pure Knowledge of the Buddha. Buddhas become Enlightened by transcending the dualism of the intellect, by realizing the ultimate unreality of all objects (dharma-nairatmya) and of empirical subjects (pudgalanairatmya), by removing the screen of suffering (kleshavarana) and of ignorance in the form of objects covering the Real (jneyavarana). Reality is silence. From that night when Buddha became Enlightened up to that night when he attained Nirvana, not a single word was uttered by him. The teaching of Buddha is truly beyond language. He who teaches with words merely babbles for Reality is beyond language and intellect. The Buddha is beyond all plurality. And that which is beyond plurality is Reality for it is beyond intellect. A finger is needed to point at the moon but the finger itself should not be mistaken for the moon. Similarly the Absolute is preached through the phenomenal, but the phenomenal should not be mistaken for the Absolute. Ultimately even this distinction is transcended. Appearances are Reality. Like Samsara and Nirvana, all things are non-dual. Reality is not to be sought apart from phenomena. Shunyavada is not nihilism...But this should not be understood in the sense of utter negation. It only means, as the Lankavatara says, that all things are unoriginated and are indescribable because they can be described as neither existent nor as non-existent nor as both. They are merely relative and so ultimately unreal. Shunya, therefore, is not merely negative. It is far better to entertain, from the empirical standpoint, an idea of Existence or Affirmation, as big as the Sumeru mountain, than to understand by 'Shunyata' a 'mere nothing'. One who maintains in a self-contradictory manner a 'mere nothing' is a self-condemned nihilist.
and pp 97-99:
Nagarjuna defines Reality (tattva) as that which can be directly realized, that which is Calm and Blissful, that where all plurality is merged, that where cries of intellect are satisfied, that which is the Non-dual Absolute. Buddha's teaching relates to two aspects of Truth--the empirical and the absolute. The first is Samvrti or Vyavahara; the second is Paramartha. Those who do not know these two standpoints cannot understand the teaching of the Buddha. Samvrti is a sort of covering. It hides the real truth. It is a workable reality, a practical makeshift, a necessary compromise. In the end it is no truth at all. But this can be realized from absolute standpoint only...
Nagarjuna explains the meaning of Shunyata. It has a double aspect. In the realm of the phenomenal it means Svabhava-shunyata or Nissvabhvata. It means that appearances are devoid of ultimate reality. It is Pratitya-samutpada or Relativity. It means that everything that can be grasped by the intellect is necessarily relative. It is the Madhyama-marga or the Middle Path between affirmation and negation--a Path which ultimately transcends both...
People, says Nagarjuna, not understanding the meaning of Shunya, accuse us of nihilism. Taking Shunya in the sense of mere negation they urge that we have negated all phenomena, that we have utterly denied the Four Noble Truths, the Bondage and Liberation, the Order, the Religion and even the Buddha, and that we have logically no room for practical compromises. We reply: These people do not understand even the meaning of Shunyata much less its real significance. Misunderstanding Shunyata in the sense of mere negation, they wrongly criticize it and charge us with defects which our doctrine does not possess. Shunyata is the negation of all views and is itself not a view. It is the realization by thought, at a higher level of diacritical self-consciousness, of its relative self-contradictory nature and of its inability to reveal the Real and an attempt to rise above and merge in spiritual experience.
The phenomenal universe rest on the Absolute. But the Absolute, according to both the Buddhist Mahayana school and the Advaita Vedanta school assert that the Absolute is not the 'cause' of the phenomenal universe; but it is that which the phenomenal universe rests upon. A desert does not 'cause' a mirage in the desert; the mirage is in the eye of the beholder, but the desert is the backdrop upon which the mirage is dependent. Being the 'cause' of the universe does not imply a first creation. Buddhists and Hindus assert that the universe has always existed and will always exist; that it goes through cycles...there was no first creation.
For an example of this same non-dualistic reasoning from an Advaita standpoint, see this link, in the Introduction pp xix-xxii, and the section following the Introduction called Adhyasa or Superimposition - https://www.wisdomlib.org/hinduism/book/brahma-sutras/d/doc62753.html