SW-824: issue warning for empty vo_event

SAS/TriggerServices/lib/trigger_service.py

@@ -242,6 +242,9 @@ class ALERTHandler(VOEventListenerInterface):
         super(ALERTHandler, self).stop_listening()
 
     def handle_event(self, voevent_xml, voevent_etree):
+        if voevent_xml is None or voevent_etree is None:
+            logger.warning("skipping empty vo_event")
+            return
 
         identifier = voevent_etree.attrib['ivorn']
         logger.info('Handling new ALERT event %s...' % identifier)