Are there any necromantic effects that target objects? I very much doubt that...
So, the ruling would be: Undead are immune to Necromantic effects, unless they specifically target Undead...
I don't know for certain, but I am pretty sure that all necromantic effects that do not specifically target Undead, exclude them from the descriton (e.g. Horrid Wilting, Finger of Death and Vampiric Touch all require living subjects).
So, all necromantic effects that cannot affect Undead do not include Undead in the target, while all those that could target them by the ruling already do. Either this "Undead are immune to Necromantic effects" thing is just plain silly or a leftover from 2nd edition maybe?