Support null as an argument where it makes sense in Gremlin.NET

We improved support for null in the last few releases, but Gremlin.NET still throws a NullReferenceExceptions if null is provided in some places, especially when it's not obvious that null is a valid argument value.

We should find these places and then either allow null there or throw an ArgumentNullException if we're really sure that null cannot be a valid argument there. In places where we don't know for sure whether null is always invalid or not, we should probably allow it in Gremlin.NET so that the server can decide how to handle it.