build_client_schema: add missing tests for introspection validations

Replicates graphql/graphql-js@37c022d

Author: Cito

README.md

@@ -13,7 +13,7 @@ a query language for APIs created by Facebook.
 [![Code Style](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
 
 The current version 1.0.1 of GraphQL-core-next is up-to-date with GraphQL.js version
-14.0.2. All parts of the API are covered by an extensive test suite of currently 1692
+14.0.2. All parts of the API are covered by an extensive test suite of currently 1701
 unit tests.
 
 

graphql/utilities/build_client_schema.py

@@ -321,12 +321,16 @@ def build_directive(directive_introspection: Dict) -> GraphQLDirective:
     # Get the root Query, Mutation, and Subscription types.
 
     query_type_ref = schema_introspection.get("queryType")
-    query_type = get_object_type(query_type_ref) if query_type_ref else None
+    query_type = None if query_type_ref is None else get_object_type(query_type_ref)
     mutation_type_ref = schema_introspection.get("mutationType")
-    mutation_type = get_object_type(mutation_type_ref) if mutation_type_ref else None
+    mutation_type = (
+        None if mutation_type_ref is None else get_object_type(mutation_type_ref)
+    )
     subscription_type_ref = schema_introspection.get("subscriptionType")
     subscription_type = (
-        get_object_type(subscription_type_ref) if subscription_type_ref else None
+        None
+        if subscription_type_ref is None
+        else get_object_type(subscription_type_ref)
     )
 
     # Get the directives supported by Introspection, assuming empty-set if directives