Create an anon class without defining fields raises `ValueError`

[felipetio]

Checklist

• [x] I have verified that that issue exists against the master branch of django-anon
• [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate
• [x] I have reduced the issue to the simplest possible case
• [x] I have included a failing test as a pull request (If you are unable to do so we can still accept the issue)

Steps to reproduce

Create an anon class without defining fields, as we can see in the README:

   import anon

   class UserAnonymizer(anon.BaseAnonymizer):
      class Meta:
         model = User

      def clean(self, obj):
         obj.set_password('test')
         obj.save()

Expected behavior

After running the anonymize the password is set to test.

Actual behavior

Using python3.9 and Django2.2 it raises an exception:

  File "/site-packages/anon/base.py", line 89, in run
    bulk_update(
  File "/site-packages/anon/compat.py", line 19, in bulk_update
    manager.bulk_update(objects, fields, **bulk_update_kwargs)
  File "/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/site-packages/django/db/models/query.py", line 491, in bulk_update
    raise ValueError('Field names must be given to bulk_update().')
ValueError: Field names must be given to bulk_update().