Make GeoFence rule removal more robust

[etj]

If GeoServer can not be reached during rule deletion, a returned None value is not properly handled:

[2024-04-11 09:15:54,091: ERROR/ForkPoolWorker-306] Error collecting rules for geonode:pat_po
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 467, in _make_request
    six.raise_from(e, None)
  File "<string>", line 3, in raise_from
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 462, in _make_request
    httplib_response = conn.getresponse()
  File "/usr/lib/python3.10/http/client.py", line 1375, in getresponse
    response.begin()
  File "/usr/lib/python3.10/http/client.py", line 318, in begin
    version, status, reason = self._read_status()
  File "/usr/lib/python3.10/http/client.py", line 279, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
  File "/usr/lib/python3.10/socket.py", line 705, in readinto
    return self._sock.recv_into(b)
TimeoutError: timed out

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/requests/adapters.py", line 486, in send
    resp = conn.urlopen(
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 799, in urlopen
    retries = retries.increment(
  File "/usr/local/lib/python3.10/dist-packages/urllib3/util/retry.py", line 550, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File "/usr/local/lib/python3.10/dist-packages/urllib3/packages/six.py", line 770, in reraise
    raise value
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 715, in urlopen
    httplib_response = self._make_request(
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 469, in _make_request
    self._raise_timeout(err=e, url=url, timeout_value=read_timeout)
  File "/usr/local/lib/python3.10/dist-packages/urllib3/connectionpool.py", line 358, in _raise_timeout
    raise ReadTimeoutError(
urllib3.exceptions.ReadTimeoutError: HTTPConnectionPool(host='geoserver', port=8080): Read timed out. (read timeout=30)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/geonode/geoserver/geofence.py", line 292, in get_rules
    r = requests.get(
  File "/usr/local/lib/python3.10/dist-packages/requests/api.py", line 73, in get
    return request("get", url, params=params, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/requests/api.py", line 59, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/requests/sessions.py", line 589, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/local/lib/python3.10/dist-packages/requests/sessions.py", line 703, in send
    r = adapter.send(request, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/requests/adapters.py", line 532, in send
    raise ReadTimeout(e, request=request)
requests.exceptions.ReadTimeout: HTTPConnectionPool(host='geoserver', port=8080): Read timed out. (read timeout=30)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/geonode/geoserver/geofence.py", line 413, in collect_delete_layer_rules
    gs_rules = self.geofence.get_rules(
  File "/usr/local/lib/python3.10/dist-packages/geonode/geoserver/geofence.py", line 307, in get_rules
    raise GeofenceException(f"Error while retrieving GeoFence rules: {e}")
geonode.geoserver.geofence.GeofenceException: Error while retrieving GeoFence rules: HTTPConnectionPool(host='geoserver', port=8080): Read timed out. (read timeout=30)
[2024-04-11 09:15:54,097: ERROR/ForkPoolWorker-306] Error removing rules for geonode:pat_po
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/geonode/geoserver/geofence.py", line 444, in delete_layer_rules
    logger.debug(f"Going to remove {batch.length()} rules for layer {workspace_name}:{layer_name}")
AttributeError: 'NoneType' object has no attribute 'length'