The Italian words could be "L'essere del nulla" or simply "il nulla". The etymology must be the same as of the English word, that is, from the Latin nullus meaning literally (ne) "not" + (ūllus) "any".
I do not know an exact Greek tradition that investigated nothingness and influenced Leonardo but I know that his environment could sometimes be described as neo-Platonic. On that trail and according to it the property is described to contain the things which it describes i.e. nothingness contains the things that do not exist.
Intuitively we describe things that are (beings) as our sensory perceptions. That is nature, everything we see and feel comes from nature and returns to it. Everything we see is also in the present, thus we know that nature is certainly in the present. We do not see things that are not. But we know things can stop being. By our past experiences we know that things we knew of are not there any more, the present, in which we make this observation and is the future of our past. By deduction we can say that things will change in the future to the point of not being as they have changed with the passing of time. Thus things that are not are found only in time and words because because of time we know that things are not and with words we can refer to them while we cannot directly experience them.
Nothingness holds the highest position and its grasp extends to
things with no being
Because things that are not are more numerous and descriptive of being than the things we immediately see. While I am not well read in regards to Greek Idealism it seems that this definition extends the typical of what a form is as the essence of an object. In the idealist sense I understand the form to be a prototype object that every true object of this kind exist in reference to. Leonardo here uses a definition that is modern and mathematical in essence. An important dimension of mathematical work consists of identifying an object under question and abstracting it as much as possible in a way that the mathematical object could describe anything of that type. I.e. 2πr and 2π*r(squared) describe the geometric properties of any circle, not a specific circle, every cirlce. A circle with r = 5 meters we might have in front of us is not the essence of the circle because it is not every circle, but just one. The essence of the circle extends to the non-being circles that every existed and will exist. That is as with the being of humans, animals and trees. More will have existed and will exist than in our present and even more are their possibilities that will never exist but still would be described as human, animal and tree. In this sense the study of topology in mathematics is relevant, but only to a certain extend, because several geometric specific properties are included. Thus we can say that being and nothingness constitutes everything and there is a mathematical intuition for that.