If any opponent has a creature card in his or her graveyard as Dawnbreak Reclaimer’s ability resolves, then you must choose one of those cards. You can’t choose a different opponent with no creature cards in his or her graveyard to avoid returning one of those cards.

If there are no creature cards in any opponent’s graveyard as Dawnbreak Reclaimer’s ability resolves, you’ll still have the option to return a creature card from your graveyard to the battlefield. You choose which opponent will choose a creature card in your graveyard.