Avoid null payload in mutation

Replicates graphql/graphql-relay-js@0595d8b

Author: Cito

Diff for: src/graphql_relay/mutation/mutation.py

@@ -31,6 +31,11 @@ def resolve_maybe_thunk(thing_or_thunk: Thunk) -> Any:
     return thing_or_thunk() if callable(thing_or_thunk) else thing_or_thunk
 
 
+class NullResult:
+    def __init__(self, clientMutationId=None):
+        self.clientMutationId = clientMutationId
+
+
 def mutation_with_client_mutation_id(
     name: str,
     input_fields: Thunk[GraphQLInputFieldMap],
@@ -76,12 +81,16 @@ async def resolve(_root, info, input):
         if isawaitable(payload):
             payload = await payload
         try:
-            payload.clientMutationId = input["clientMutationId"]
+            clientMutationId = input["clientMutationId"]
         except KeyError:
             raise GraphQLError(
                 "Cannot set clientMutationId"
                 f" in the payload object {inspect(payload)}."
             )
+        if payload is None:
+            payload = NullResult(clientMutationId)
+        else:
+            payload.clientMutationId = clientMutationId
         return payload
 
     return GraphQLField(

Diff for: tests/mutation/test_mutation.py

@@ -178,6 +178,21 @@ async def can_access_root_value():
             None,
         )
 
+    @mark.asyncio
+    async def supports_mutations_returning_null():
+        query = """
+          mutation M {
+            simpleRootValueMutation(input: {clientMutationId: "abc"}) {
+              result
+              clientMutationId
+            }
+          }
+        """
+        assert await graphql(schema, query, root_value=None) == (
+            {"simpleRootValueMutation": {"result": None, "clientMutationId": "abc"}},
+            None,
+        )
+
     def describe_introspection():
         @mark.asyncio
         async def contains_correct_input():