fix uninitialized value of msg

if a user leaves or quits without a message, the following shows up in the error log:

hggh quit:
Use of uninitialized value $msg in concatenation (.) or string at lib/Hubot/Adapter/Irc.pm line 153.

hggh leaves:
Use of uninitialized value $msg in concatenation (.) or string at lib/Hubot/Adapter/Irc.pm line 145.