Handle full_text=None, "expected string or buffer" crash from other bots' messages

[grschafer]

Other bots' messages may contain None for the message text, such as in the example below:

{
  'event_ts': '1490370300.598923',
  'username': 'removed',
  'type': 'message',
  'team': 'T...',
  'ts': '1490370300.598923',
  'mrkdwn': True,
  'channel': 'G...',
  'text': None,
  'subtype': 'bot_message',
  'bot_id': 'B...',
  'attachments': [{
    'color': 'ac725e',
    'mrkdwn_in': ['text', 'pretext'],
    'title_link': 'https://www.google.com/calendar/event?eid=NH...',
    'title': 'testing slackbot', 'pretext': 'Event starting in 30 minutes:',
    'fallback': 'testing slackbot: <!date^1490372100^{date_short_pretty} from {time}|Mar 24, 2017 from 10:15 AM> to <!date^1490375700^{time}|11:15 AM GMT-0600>',
    'text': '<!date^1490372100^{date_short_pretty} from {time}|Mar 24, 2017 from 10:15 AM> to <!date^1490375700^{time}|11:15 AM GMT-0600>',
    'id': 1
  }]
}

The text field being None makes msg.get('text', '') return None instead of an empty string, which subsequently crashes the regex match line. Traceback below:

Traceback (most recent call last):
  ...
  File "myscript.py", line 84, in main
    bot.run()
  File "/usr/local/lib/python3.4/dist-packages/slackbot/bot.py", line 37, in run
    self._dispatcher.loop()
  File "/usr/local/lib/python3.4/dist-packages/slackbot/dispatcher.py", line 143, in loop
    self._on_new_message(event)
  File "/usr/local/lib/python3.4/dist-packages/slackbot/dispatcher.py", line 93, in _on_new_message
    msg_respond_to = self.filter_text(msg)
  File "/usr/local/lib/python3.4/dist-packages/slackbot/dispatcher.py", line 110, in filter_text
    m = self.AT_MESSAGE_MATCHER.match(full_text)
TypeError: expected string or buffer

This PR just adds another fallback, so None gets turned into an empty string to avoid crashing.

[jtatum]

Thanks for the PR 👍