If the prior probability of every universal hypothesis is zero how would you have to rate the probability of the statement that unicorns (at least one) exist?
Probability of an event = number of outcomes that would fall under this event / total possible number of outcomes.
To say that prior probability of every universal hypothesis is zero is Popper's view that total possible number of outcomes is infinite (thus 1/infinity = 0). No matter how many theories you falsify, there are an infinite number of theories that have not been tested, and so your remaining theories out of this infinite possibility space is zero.
But what about a verifiable existential claim like "at least 1 unicorn exists" be probabilistically assessed?