Fix error when nick is null

You are checking if PASS is null but not NICK, I was getting an error on [line 695](https://github.com/jisaacks/node-irc/blob/master/lib/irc.js#L695):
```javascript
if ( args[args.length-1].match(/\s/) || args[args.length-1].match(/^:/) || args[args.length-1] === "" ) {
```
The error was:
>TypeError: Cannot call method 'match' of null

Because `self.opt.nick` was null

I just added the same logic to not send nick if it is null.