Issue with nested serializers: expected iterable but did not find one

Running into similar issue to these issues:
- https://github.com/graphql-python/graphene-django/issues/371
- https://github.com/graphql-python/graphene-django/issues/458

This was supposed to be resolved in https://github.com/graphql-python/graphene-django/pull/395

I am on version`2.13.0`

I have a nested serialized using `many=True` for the related serializer, everything saves fine, but on return I am getting:

```
File "/../graphql/execution/executor.py", line 531, in
lambda resolved: complete_value(
File "/../graphql/execution/executor.py", line 566, in complete_value
return complete_list_value(
File "/../graphql/execution/executor.py", line 603, in complete_list_value
assert isinstance(result, Iterable), (
AssertionError: User Error: expected iterable, but did not find one for field MyObjectMutationPayload.objects.
```

This seems to imply there is an issue with the return data.