feat: support nullable graphql query

[huuyafwww]

Hello, thank you for developing this wonderful library called msw. I am using msw in my graphql project. There I have a Query with nullable return value.
And I generate that type using graphql-codegen.
Here is an example of the generated type.

export type Maybe<T> = T | null;

export type IQuery = {
  __typename?: 'Query';
  authUser: Maybe<IUser>;
  user: Maybe<IUser>;
};

And I tried to use this query type which may return null in graphql.query and graphql.mutation.

import { graphql } from 'msw';

graphql.query<IQuery['authUser'], /* variable type */>(queryName, () => {});
graphql.mutation</* nullable mutation */, /* variable type */>(queryName, () => {});

Then I got a type error that nullable queries and mutations are not supported.

I would like to have the actual value returned by a query or mutation set strictly as a type in mock as well.
Therefore, I have modified GraphQLQuery to allow null.
What do you think?