Fix a crash bug when a zero length message is received

Hello.

When a zero-length PRIVMSG / NOTICE message is sent to an channel that a node-irc bot is present in, it crashes with the following backtrace:

```
/home/shiwano/projects/lo_Ol/node_modules/hubot-irc/node_modules/irc/lib/irc.js:637
                    throw err;
                          ^
TypeError: Cannot read property '0' of undefined
  at Client.<anonymous> (/home/shiwano/projects/lo_Ol/node_modules/hubot-irc/node_modules/irc/lib/irc.js:468:25)
  at Client.EventEmitter.emit (events.js:95:17)
  at /home/shiwano/projects/lo_Ol/node_modules/hubot-irc/node_modules/irc/lib/irc.js:634:22
  at Array.forEach (native)
  at CleartextStream.<anonymous> (/home/shiwano/projects/lo_Ol/node_modules/hubot-irc/node_modules/irc/lib/irc.js:631:15)
  at CleartextStream.EventEmitter.emit (events.js:95:17)
  at CleartextStream.<anonymous> (_stream_readable.js:720:14)
  at CleartextStream.EventEmitter.emit (events.js:92:17)
  at emitReadable_ (_stream_readable.js:392:10)
  at _stream_readable.js:385:7
  at process._tickCallback (node.js:415:13)
```

I fixed it. Could you review it? Thanks.