Fix ArrayAgg() behavior for django 5

[bctcvai]

From unit tests:

  /usr/local/lib/python3.10/dist-packages/django/contrib/postgres/aggregates/general.py:45: RemovedInDjango50Warning: In Django 5.0, ArrayAgg() will return None instead of an empty list if there are no rows. Pass default=None to opt into the new behavior and silence this warning or default=[] to keep the previous behavior.
    warnings.warn(self.deprecation_msg, category=RemovedInDjango50Warning