Fix __type queries sometimes not returning data

[simhnna]

If the queried type was not defined it would return an empty data section instead of __type: null

Fixes #539

[pavelnikolov]

@simhnna thank you for opening the issue and contributing with a PR. Does the response in the current version of the library actually violate the GraphQL Spec?

[simhnna]

https://spec.graphql.org/October2021/#sec-Schema-Introspection
Defines __type(name: String!): __Type
So the return type is nullable.

And https://spec.graphql.org/October2021/#sec-Executing-Fields
states that

Each field requested in the grouped field set that is defined on the selected objectType will result in an entry in the response map.

So yes I would say the current implementation violates the spec. I know that Apollo server returns a field with null value for such queries. If you want I can double check the JavaScript reference implementation tomorrow on my work PC, but I'm pretty certain it also returns a field with a null if asked for an unknown type.