Support `null` literal for optional fields

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 https://github.com/facebook/graphql/pull/83

Start by tracing how mutation variables are serialized for optional fields in the Android SDK, then compare that behavior with GraphQL's null literal semantics described in the linked specification change. Done means an explicitly supplied null is preserved and sent rather than treated as an omitted field, with regression coverage for the mutation case.

Written by the indexing model from the issue text.