lack of jabber password yields an unexpected exception

When you forget to enter a jabber password, you get that crazy error in stderr:

INFO source.jabber: Connecting to: hermes.jabber.org:5222
2010-08-16 15:47:00-0400 [XmlStream,client] Unhandled Error
        Traceback (most recent call last):
          File "/build/buildd-python2.5_2.5.2-15+lenny1-amd64-gBDyED/python2.5-2.5.2/Modules/pyexpat.c", line 656, in EndElement

          File "/usr/lib/python2.5/site-packages/twisted/words/xish/domish.py", line 780, in _onEndElement
            self.ElementEvent(self.currElem)
          File "/usr/lib/python2.5/site-packages/twisted/words/xish/xmlstream.py", line 106, in onElement
            self.dispatch(element)
          File "/usr/lib/python2.5/site-packages/twisted/words/xish/utility.py", line 316, in dispatch
            callbacklist.callback(obj)
        --- <exception caught here> ---
          File "/usr/lib/python2.5/site-packages/twisted/words/xish/utility.py", line 106, in callback
            methodwrapper(*args, **kwargs)
          File "/usr/lib/python2.5/site-packages/twisted/words/xish/utility.py", line 27, in __call__
            self.method(*nargs, **nkwargs)
          File "/usr/lib/python2.5/site-packages/twisted/words/protocols/jabber/sasl.py", line 193, in onChallenge
            self.sendResponse(self.mechanism.getResponse(challenge))
          File "/usr/lib/python2.5/site-packages/twisted/words/protocols/jabber/sasl_mechanisms.py", line 100, in getResponse
            directives['nonce'])
          File "/usr/lib/python2.5/site-packages/twisted/words/protocols/jabber/sasl_mechanisms.py", line 191, in _gen_response
            password = self.password.encode(charset)
        exceptions.AttributeError: 'NoneType' object has no attribute 'encode'

That seems a bit counter-intuitive... We should do a quick check on self.password and send a friendly message if None...