If I use Castform's "Hurricane" attack and I have no benched Pokemon, do I have to discard an Energy card?
No, if you have no Benched Pokemon all Energy remains attached to Castform.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_116
SWSH9_116