Can I use Hatterene's "Mind Hat" Ability if I don't have any cards in my hand? Or if my opponent doesn't have any cards in their hand?
As long as either player has a card in their hand, you can use Mind Hat.
Source: Rebel Clash FAQ (2020-04-18), TPCi Rules Team (2020-04-30)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH2_85
SWSH2_85