Quote Originally Posted by Kyutaru View Post
Now what made you think that? Revenants in 3.5 have Regeneration 10. Regeneration is definitely an allowed Null Constitution ability. Additionally, Vampires have Fast Healing which operates similarly but is a better version of it.
Revenants are probably a mistake, then. Note the last line in this linked subsection: "A creature must have a Constitution score to have the regeneration ability."

Vampires have Fast Healing precisely because they can't have regeneration.

Regeneration actually is better than Fast Healing for most purposes: it keeps you from taking lethal damage from anything other than that which penetrates your regeneration. Creatures without Constitution scores are immune to nonlethal damage, which is why they're not allowed to have regeneration and have to have fast healing instead.