Add support for null vs missing input value

[Ackar]

This is an attempt to solve #210 by adding support for nullable non-pointer types. This is inspired by the Null types in the sql package.

The idea is to have custom types where we can call UnmarshalGraphQL with a nil input, thus allowing to differentiate between no value (method not called) and a null value (method called with nil).

The main change is to allow a non-pointer type to represent a nullable value if it implements the following interface:

type NullUnmarshaller interface {
	Unmarshaler
	Nullable()
}

Also added nullable types for existing scalars: NullString, NullInt, NullFloat, NullBool and NullTime.

Example

input MyInput {
    a: String
}

type Query {
    myQuery(input: MyInput!): String!
}

type MyInput struct {
	A graphql.NullString
}

type resolver struct{}

func (r *resolver) MyQuery(args struct {
	Input MyInput
}) string {
	if args.Input.A.Valid {
		if args.Input.A.Value == nil {
			// null value provided
		} else {
			// non-null value provided
		}
	} else {
		// field omitted
	}

	return "hello world"
}

[pavelnikolov]

Hi @Ackar
Thank you for your contribution. I'll take a closer look tomorrow.

[pavelnikolov]

Thank you