To clarify ramolnar's comment about Galadriel:
Galadriel, Lady of Light should be able to exert for her ability even if you do not have a playable Elf in hand. (This is similar to the way Cantea can exert when his opponent bears no weapon or even if he is not involved in a skirmish (which is allowed), you can play
Secret Sentinels even if there are no shadow conditions (also allowed), etc.)
I haven't seen it, but the same bug probably affects
Gandalf, The Grey Wizard if you have no playable companions in hand.
(This is relevant in case you have an extra Galadriel or Gandalf in hand and want to cycle the card to heal them.)