I’m my opponent plays emperor’s divide and chooses to move all his units back to base, can I play Rengar Ambush to that battlefield in the chain? (essentially playing into an empty field).

Yes. You can play Rengar, Trophy Hunter to that battlefield in response on the chain — because Emperor's Divide hasn't resolved yet, enemy units are still there when you choose the location and when Rengar finalizes (Rule 822.3).

The chain resolves LIFO: Rengar enters first, then Emperor's Divide moves the opponent's units to base. Rengar isn't a "friendly unit" for their spell, so he stays — leaving the field with only your Rengar. If the enemy units were already gone (i.e., you tried to play him after Emperor's Divide resolved), his Ambush permission would fail since no enemy units remain (Rule 822.3). Note this only works with Trophy Hunter — Rengar, Pouncing needs you to be the attacker, which you aren't here.