You may cast Dark Salvation with X as 0 just to have a creature get -1/-1 for each Zombie a player controls, and you may cast it without targeting any creature just to have a player get Zombie tokens. If you really just want to cast a spell, you can even cast it with X as 0 without a target creature and have neither effect do anything.

You choose up to one target creature total, not a separate target for each Zombie that player controls.

If the targeted creature becomes an illegal target, the targeted player will still get Zombie tokens.