Make errors in form mutation non nullable (#1286)

Author: artofhuman

graphene_django/forms/mutation.py

@@ -117,7 +117,7 @@ class DjangoModelFormMutation(BaseDjangoFormMutation):
     class Meta:
         abstract = True
 
-    errors = graphene.List(ErrorType)
+    errors = graphene.List(graphene.NonNull(ErrorType), required=True)
 
     @classmethod
     def __init_subclass_with_meta__(