If I use Tropius' "Tropic Breeze" attack and I don't have any benched Pokemon, what happens to the energy that is attached to Tropius?
It stays attached to Tropius.
Source: Paldea Evolved FAQ; TPCi Rules Team (2023-06-08)

Relevant cards

Note: All applicable cards may not be listed below.
SV2_7
SV2_7