If there are no legal targets that could be selected, the targets remain unchanged. Note that this may not mean that those targets are illegal as the spell resolves. For example, if an opponent in a two-player game has hexproof and casts Lava Spike targeting you, Chef's Kiss will not be able to change the target. However, this doesn't make you an illegal target, and both the original spell and the copy will still target you and deal 3 damage to you.