Be careful with your definition of LC. Lombre is also a Pokemon that can still evolve, but you can't use Lombre in LC, either. To be an eligible member of LC, each Pokemon must also:
- be the first stage in its evolution family, which Type: Null is, and
- be obtainable at level 5 in the game, which Type: Null is not.
You cannot get a level 5 Type: Null though normal gameplay, so it isn't legal according to LC rules. (This is because neither Type: Null nor Silvally can breed and Type: Null has never been distributed at a level <40.)
Also, Type: Null would be banned from Smogon LC if it were ever eligible for the format.