The spell description says it can't affect undead unless they're destroyed, but the description of the undead type in the MM says that an undead targetted by true res' turns back into a living creature.
Ask your DM.
You are right, I never noticed it before.
It seems that you still can't use True Res the undead is (un)alive but not the target of the spell, though, so the point is somewhat the same.
Ignotus Peverell avatar made by the great Bradakhan.