Support serialise nested object as null

[see-mack]

The Android client is facing the same problem as iOS. See Shopify/graphql_swift_gen#7

[wesleyjellis]

@eapache I could have sworn that we had support for this already somewhere but I can't seem to find us using it

[eapache]

I think you're thinking of the other case: telling the difference between a field being missing or having a literal null when returned from the server. I don't think we ever supported it on input objects.

[dylanahsmith]

We didn't used to support it, since the GraphQL standard didn't used to allow a literal null value. That was a more recent addition.