Support for empty literals, strings and groups

[ErisDS]

Originally reported at TryGhost/GQL#36

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See: TryGhost/Ghost#9258 & TryGhost/Casper#382

I've seen this pop up a few times. If you use dynamic queries, there's no guarantee that you will get anything output, and then NQL will throw a parse error.

Short term the solution is to put guards around dynamic queries, but I think we can do better in NQL Lang, by handling the concept of an empty clause, and removing the whole clause if empty.