don't escape null argument

I've added a special case in order to prevent grex from escaping null arguments. This allows queries such as `db.V().has('name', 'T.neq', null)`.

As as note, I thought it made sense to support the JavaScript null object as opposed to a 'null' string to prevent any ambiguities. Although, it's a bit odd that it doesn't follow the convention of the other Java values (e.g. T.eq).