Add check to see if prefix is None in on_message

This was causing crashes, I'm not sure why prefix is coming back empty

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

omnibot/bot.py

@@ -190,6 +190,8 @@ class Bot:
         log.trace("%s", message)
         channel = message.parameters[0]
         who = message.prefix
+        if not who:
+            return
         if who.nick == self.server_config.nick:
             # Don't raise on_message events for ourselves
             return