execute: Forbid to return null from serialize function

Replicates graphql/graphql-js@0fef3c4

Author: Cito

src/graphql/execution/execute.py

@@ -769,10 +769,10 @@ def complete_leaf_value(return_type: GraphQLLeafType, result: Any) -> Any:
         serialization is not possible.
         """
         serialized_result = return_type.serialize(result)
-        if serialized_result is Undefined:
+        if serialized_result is Undefined or serialized_result is None:
             raise TypeError(
-                f"Expected a value of type '{inspect(return_type)}'"
-                f" but received: {inspect(result)}"
+                f"Expected `{inspect(return_type)}.serialize({inspect(result)})`"
+                f" to return non-nullable value, returned: {inspect(serialized_result)}"
             )
         return serialized_result
 

tests/execution/test_executor.py

@@ -983,8 +983,8 @@ def fails_when_serialize_of_custom_scalar_does_not_return_a_value():
             {"customScalar": None},
             [
                 {
-                    "message": "Expected a value of type 'CustomScalar'"
-                    " but received: 'CUSTOM_VALUE'",
+                    "message": "Expected `CustomScalar.serialize('CUSTOM_VALUE')`"
+                    " to return non-nullable value, returned: Undefined",
                     "locations": [(1, 3)],
                     "path": ["customScalar"],
                 }