Support `null` literal for optional fields

[paulfthomas]

Not sure if it's a bug or a missing feature but null literals don't seem to work.
Passing null to an optional field for a mutation results in the field being ignored.

It's been added to GraphQL, see graphql/graphql-spec#83

[minbi]

Hi @paulfthomas ,

We'll need to look into this. Thanks for the report.