Use ? for nullable fields

[timsuchanek]

If you have a type that includes an optional field, the following is generated right now:

export interface PlaceRoot {
  id: string
  name: string
  size: PLACE_SIZES | null
  maxGuests: number
}

This means that the resolver needs to return an explicit null value:

const resolvers = {
  size: () => null
}

However, graphqljs allows returning undefined for a nullable value.
This makes implementations easier, as we don't need to return an explicit null anymore.

To make this possible, the only difference is turning | null into ?:

export interface PlaceRoot {
  id: string
  name: string
  size?: PLACE_SIZES
  maxGuests: number
}

I just tested it in TypeScript 2.9.2 - ? allows both undefined and null.