Rule 508.4b

If the effect that states a creature is attacking specifies it’s attacking a certain player, and that player is no longer in the game when the effect resolves, the creature doesn’t become an attacking creature. The same is true if the effect specifies a creature is attacking a planeswalker or battle and, when the effect resolves, that permanent is no longer on the battlefield, is no longer a planeswalker or battle, is a planeswalker that is no longer controlled by a defending player, or is a battle that is no longer protected by a defending player.