Support null literal

[jonricaurte]

The GraphQL language added null literal support a few months ago and it has not been added to graphql-go:

graphql/graphql-spec#83

[bbuck]

@seriallink That was an accident on my part, I'm sorry. The body of the comment was:

+1

But in the future, please refrain from commenting things like that as there is the reactions you can add to other comments (as of this comment here 8, including myself, have reacted with +1 and/or thumbs up).

[MTigra]

It is still not fixed:(
https://stackoverflow.com/questions/49422706/golang-graphql-fails-to-tell-nil-pointer-from-nil-literal

[david4shure]

Any movement on this? @chris-ramon @sogko @lenaten

[mpenick]

Added a fairly well tested PR for this #536.