Prevent property(id,null) and addV(null)

for g.addV().property(id,null) isn't quite the same as the String key version because the id can't be null and it can't behave like a removal for graphs that don't support null values. allow this might also create hard to figure out bugs where a variable for the id gets set to null accidentally and random data gets created rather than getting an error. There is a similar argument to be made for addV(null).

at this time, there is a test that enforces this behavior so to remove that changes behavior which means we probably need to save this for a breaking change.