In my opinion it doesn't seem to be based on anything in real life directly. Actually, its rumored to be a failed clone of Arceus like how Mewtwo (maybe Ditto) are clones of Mew.
I think Type: Null is a failed attempt to recreate Arceus scientifically. Aside from the locking mechanism on it's neck (being in the shape of arceus's ring), the giant triangular thing on top of its head is remarkably similar to arceus's long, whispy thing on the top of his. Also notice how Type: Null is attempting to essentially be almost every other type in its design. Arceus can be any type also. It may be a weird side-effect of trying to re-create the multi-type ability in another Pokemon. This poses the question, who actually has the ability and/or equipment to create such a beast? I would argue that is would be the Aether Foundation. Team skull just doesn't have the resources and team flare hasn't been seen as present in this gen. I dunno. Way too many coincidences to not have a connection IMO.
-Spark31 from reddit
Also, it's called the Synthetic Pokémon. Synthetic means made by chemical synthesis, especially to imitate a natural product.
Lastly, it's evolution's ability: RKS System
> Sun/Moon Changes the Pokémon’s type to match the memory disc it holds.
Doesn't that sound familiar?
HeartGold/SoulSilver Changes type to match the held Plate.
Sun/Moon Changes the Pokémon’s type to match the Plate or Z-Crystal it holds.
In conclusion, I believe that Null-Type/Silvally is a failed (idk if its failed) clone of Arceus. But you were asking, what Null-Type based on in real life, so here is the pokedex entry of Arceus instead:
> Diamond It is described in mythology as the Pokémon that shaped the universe with its 1,000 arms.
Pearl It is told in mythology that this Pokémon was born before the universe even existed.
Omega Ruby According to the legends of Sinnoh, this Pokémon emerged from an egg and shaped all there is in this world.
Alpha Sapphire It is told in mythology that this Pokémon was born before the universe even existed.
It's basically the god in the Pokemon world. So to wrap it up: Null-Type is based on Arceus; Arceus is based on God (or whatever you believe in I guess).