I suppose, in the unlikely event that the Boons deck and discard pile are both totally empty when you're supposed to receive a Boon, nothing happens?
When I first read this, I was trying to figure out how this could actually happen. 3 get set aside in setup for Druid. 3 of them (The Field's Gift, The Forest's Gift, and The River's Gift) go in front of a player until that turn's Clean-up, so if you either have a great engine going with lots of villages, or play a bunch of Pixies (the only Fate card, other than Idol that's non-terminal), you could get those 3 also set aside. But then there's still 6 more. But the rest, when you receive them, go immediately into the Boon discard pile, so even playing the Fool, receiving 3, doesn't empty the pile.
Then I looked through the set and found what I believe is the only other way to get Boons set aside - gaining Blessed Village (and choosing to receive them at the start of your next turn). Ok, so that will need a ton of money and a bunch of buys to trigger all those Boons, since you're now in your Buy phase and can't rely on playing Fool.
Then it dawned on me... Ironworks! Play a bunch of those, gaining Blessed Village each time. Then you're still in your Action phase to play Fool. All in all, fairly accessible now.