anyway, reading sankaras bhashya on the vedanta sutras, he does say that existence comes from existence, aka sat comes from sat, aka truth from truth. i could be stretching the wording, of course what he meant was to refute creation from nothing. anyway he also says Brahman has characteristics, otherwise it could not be contemplated, which seems to controvert his belief in nirguna brahman, but actually does not. imo nirguna brahman is nirguna because it is ineffable, indiscrete, and contains everything as the Truth (so there is nothing that is not like it in the higher sense), thus transcending consideration of quality. but nirguna brahman still can be described through mere discrete words as pure, infinite, partless, non-dual, existence, consciousness, bliss.