v12-upgrade: community gives 500 if first available owner is a group

[zzacharo]

Package version (if known): v12.0.0.0b3.x

Describe the bug

When upgrading to v12, communities having an owner a group fail to be displayed. If the first owner is a user then a group can be added as an owner. This happens because groups do not have a verified_at column, so it fails here.

Expected behavior

We should make the code more resilient and allow groups to be the only owners of a community.

Additional context

Relevant exception

KeyError: 'is_verified'
  File "marshmallow/utils.py", line 293, in _get_value_for_key
    return obj[key]
KeyError: 'user'
  File "flask/app.py", line 2529, in wsgi_app
    response = self.full_dispatch_request()
  File "flask/app.py", line 1825, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "flask/app.py", line 1823, in full_dispatch_request
    rv = self.dispatch_request()
  File "flask/app.py", line 1799, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "invenio_communities/views/decorators.py", line 32, in view
    request.community = community.to_dict()
  File "invenio_communities/communities/services/results.py", line 116, in to_dict
    res = self.data
  File "invenio_records_resources/services/records/results.py", line 71, in data
    self._data = self._schema.dump(
  File "invenio_records_resources/services/records/schema.py", line 105, in dump
    return self.schema(context=context, **schema_args).dump(data)
  File "marshmallow/schema.py", line 549, in dump
    result = self._serialize(processed_obj, many=many)
  File "marshmallow/schema.py", line 517, in _serialize
    value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
  File "marshmallow/fields.py", line 332, in serialize
    value = self.get_value(obj, attr, accessor=accessor)
  File "marshmallow/fields.py", line 263, in get_value
    return accessor_func(obj, check_key, default)
  File "marshmallow/schema.py", line 476, in get_attribute
    return get_value(obj, attr, default)
  File "marshmallow/utils.py", line 276, in get_value
    return _get_value_for_key(obj, key, default)
  File "marshmallow/utils.py", line 295, in _get_value_for_key
    return getattr(obj, key, default)
  File "invenio_records_resources/records/systemfields/calculated.py", line 47, in __get__
    return self.obj(record)
  File "invenio_records_resources/records/systemfields/calculated.py", line 29, in obj
    return self.calculate(instance)
  File "invenio_communities/communities/records/systemfields/is_verified.py", line 28, in calculate
    if owner["user"]["verified_at"] is not None: