Can I use Swampert's "Power Draw" Ability to discard a card from my hand if there are no cards left in my deck?
Yes you can. The first part of Power Draw is to discard a card from your hand, period. Then you do as much of the rest of the Ability as you are able to.
Source: TPCi Rules Team (2019-03-21)

Relevant cards

Note: All applicable cards may not be listed below.
SM7_35