Impossible to create a corpus in production

Also Reported in https://gitlab.com/arkindex/requests/-/issues/221

Sentry Issue: ARKINDEX-BACKEND-7Q

TypeError: '>=' not supported between instances of 'NoneType' and 'int'
(10 additional frame(s) were not displayed)
...
  File "rest_framework/serializers.py", line 562, in data
    ret = super().data
  File "rest_framework/serializers.py", line 260, in data
    self._data = self.to_representation(self.instance)
  File "rest_framework/serializers.py", line 529, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "rest_framework/fields.py", line 1905, in to_representation
    return method(value)
  File "arkindex/documents/serializers/elements.py", line 104, in get_rights