Fix handling of Clair notifications without New block

Fixes #1398

util/secscan/notifier.py

@@ -18,6 +18,10 @@ def process_notification_data(notification_data):
   """ Processes the given notification data to spawn vulnerability notifications as necessary.
       Returns whether the processing succeeded.
   """
+  if not 'New' in notification_data:
+    # Nothing to do.
+    return True
+
   new_data = notification_data['New']
   old_data = notification_data.get('Old', {})