add `NullValueNode` (#13152)
add `NullValueNode` (#13152)

In the most recent update to GraphQL, the null literal was added to the AST. This PR adds the types for the new AST node. For reference, see:

- https://github.com/graphql/graphql-js/blob/e49df4980396ed40d9b43bad738c53e001d8f0e5/src/language/ast.js#L129
- https://github.com/graphql/graphql-js/blob/e49df4980396ed40d9b43bad738c53e001d8f0e5/src/language/ast.js#L264
- https://github.com/graphql/graphql-js/blob/e49df4980396ed40d9b43bad738c53e001d8f0e5/src/language/ast.js#L293-L296
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