It sounds more to me that this is half literal and half figurative. So… Let’s start with one definition of “Witness”, which is:
One present at a transaction so as to be able to testify to its having taken place.
So… literally it would be saying that the sky is the entity that can attest or testify to what he is claiming. But, that doesn’t make much sense in a literal sense. So… take one more step to view this middle statement as symbolic. The clear sky would most likely represent everything above it as well, peering down upon him (aka. the universe).
Kerouac is calling upon the universe to testify upon his behalf, to declare the things it has seen that he is claiming. The “empty sky” indicates a clear view as well as an emptiness that doesn’t seem to end or possibly does not end.
If we put all of these ideas together it brings a hazy visual of the universe peering down upon him clearly through a vast void and seeing him and his actions in a way that no one else does or could, because it was there… and watching without obstruction. I hope this is helpful!