Can I trigger Sunken Temple on Rengar pouncing if there are no enemy units on the battlefield?

No, you cannot trigger Sunken Temple's ability on Rengar, Pouncing when moving to an empty battlefield.

Sunken Temple's conquer trigger checks for "one or more Mighty units" at the exact moment the conquer is established (FAQ #9815). Rengar's base Might is 3, and his Assault 2 (+2 while an attacker) only applies when he has the Attacker designation. But when you move to an empty battlefield, it's a Non-Combat Showdown (Rule 348.2) — not a Combat. The Attacker/Defender designations are only established "when Combat opens" (Rule 459.2.b), and since there are no enemy units, no Combat ever opens. Rengar never gains the Attacker designation, so Assault 2 never kicks in, and he stays at 3 Might — not Mighty (needs 5+).

If there were enemy units at the battlefield (making it a Combat Showdown), Rengar would gain the Attacker designation at the start of combat, Assault 2 would bump him to 5 Might, and Sunken Temple would trigger on conquer. But with no enemies, the ability fizzles.