Adding check for None type object in date fields.

mq/plugins/guardDuty.py

@@ -59,7 +59,10 @@ class message(object):
         # reformat the date fields to iosformat
         for date_key in self.date_keys:
             if key_exists(date_key, message):
-                message = self.convert_key_date_format(date_key, message)
+                if message.get(date_key) == None:
+                    continue
+                else:
+                    message = self.convert_key_date_format(date_key, message)
 
         # convert the dict to a dot dict for saner deep key/value processing
         message = DotDict(message)