Nether Void's ability doesn't trigger as any player's upkeep begins, nor does it affect creatures on the battlefield (C.R. 108.1).
If you are confusing that ability with that of Braids, Cabal Minion, note that the latter ability doesn't target anything (C.R. 115.1d), so that when that ability resolves, the appropriate player can choose even a creature with hexproof to sacrifice (and must sacrifice that creature if they control no other creatures, no other artifacts, and no other lands) (review C.R. 702.11b).
EDIT (Oct. 29): Edit citation of one rule in view of Core Set 2020 update.
If you are confusing that ability with that of Braids, Cabal Minion, note that the latter ability doesn't target anything (C.R. 115.1d), so that when that ability resolves, the appropriate player can choose even a creature with hexproof to sacrifice (and must sacrifice that creature if they control no other creatures, no other artifacts, and no other lands) (review C.R. 702.11b).
EDIT (Oct. 29): Edit citation of one rule in view of Core Set 2020 update.