Are there any rules in Star Realms for when you have no deck?
This can happen in an actual game if a person trashes all of their economy in order to get more damage efficiency.  A person with no money in their deck cannot purchase anything since the cheapest card in the market costs 1 (scouts are not in the market).  This is different from Dominion, where you can buy copper for 0.

Additionally, if you have fewer than five cards in your deck at the end of a turn (either because of trashing or because your deck is mostly bases and you have all of them out), you draw as many cards as you can (namely all the cards in your deck).