Travis R. Chance
I'm not certain it's mathematically possible, actually. In any case it never even came close to happening in playtesting. If it does happen in your game, please tell us about it! And just shuffle your discards to make a new draw deck.
In the many, many, many months of testing and development of this game, we never had a player's deck empty. Perhaps there is a perfect storm where it could happen, wherein we would suggest shuffle up the discard and make a new deck.