Remove non-null restriction on clientMutationId field definitions

Replicates graphql/graphql-relay-js@f2aee93

see graphql/graphql-relay-js#79

Author: Cito

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

@@ -62,13 +62,13 @@ def mutation_with_client_mutation_id(
     def augmented_input_fields() -> GraphQLInputFieldMap:
         return dict(
             resolve_maybe_thunk(input_fields),
-            clientMutationId=GraphQLInputField(GraphQLNonNull(GraphQLString)),
+            clientMutationId=GraphQLInputField(GraphQLString),
         )
 
     def augmented_output_fields() -> GraphQLFieldMap:
         return dict(
             resolve_maybe_thunk(output_fields),
-            clientMutationId=GraphQLField(GraphQLNonNull(GraphQLString)),
+            clientMutationId=GraphQLField(GraphQLString),
         )
 
     output_type = GraphQLObjectType(name + "Payload", fields=augmented_output_fields)

Diff for: tests/star_wars_schema.py

@@ -78,13 +78,13 @@
 # }
 #
 # input IntroduceShipInput {
-#   clientMutationId: string!
+#   clientMutationId: string
 #   shipName: string!
 #   factionId: ID!
 # }
 #
-# input IntroduceShipPayload {
-#   clientMutationId: string!
+# type IntroduceShipPayload {
+#   clientMutationId: string
 #   ship: Ship
 #   faction: Faction
 # }
@@ -201,13 +201,13 @@ def get_node_type(obj, _info, _type):
 #
 # It creates these two types implicitly:
 #   input IntroduceShipInput {
-#     clientMutationId: string!
+#     clientMutationId: string
 #     shipName: string!
 #     factionId: ID!
 #   }
 #
-#   input IntroduceShipPayload {
-#     clientMutationId: string!
+#   type IntroduceShipPayload {
+#     clientMutationId: string
 #     ship: Ship
 #     faction: Faction
 #   }