bots: Correct weird behavior of followup bot for empty messages.

Before it sends an empty message to followup stream, now it sends a
help message back to the user/stream where it was mentioned.

contrib_bots/bots/followup/followup.py

@@ -23,6 +23,10 @@ class FollowupHandler(object):
             '''
 
     def handle_message(self, message, client, state_handler):
+        if message['content'] == '':
+            bot_response = "Please specify the message you want to send to followup stream after @mention-bot"
+            client.send_reply(message, bot_response)
+        else:
             bot_response = self.get_bot_followup_response(message)
             client.send_message(dict(
                 type='stream',