adding check for nullity in related models delete

[AllenWang314]

When related models are deleted, the block of code here around models.py:234 gives the error:

File "/workspaces/project/.venv/lib/python3.9/site-packages/safedelete/models.py", line 246, in soft_delete_cascade_policy_action
_, delete_response = res
TypeError: cannot unpack non-iterable NoneType object

Based on the context, seems like this bug comes about because the original object that is being deleted appears in the related_objects(self) function.

      deleted_counter: Counter = Counter()
        for related in related_objects(self):
            if is_safedelete_cls(related.__class__) and not getattr(related, FIELD_NAME):
                res = related.delete(force_policy=SOFT_DELETE, is_cascade=True, **kwargs)
                if res is not None:
                    _, delete_response = res
                    deleted_counter.update(delete_response)

This issue appeared in django = "4.1.7", the latest version of django-safedelete as well as 1.3.3. Python version is Python 3.9.20. This is the exact bug in #235

[Gagaro]

Thanks for the PR 👍