Check whether a value is nil that may be set nil in parsing IRC message'...

```ruby
join_msg = ":example!~example@example.com JOIN :#channel\r\n"
Zircon::Message::Patterns::MESSAGE_PATTERN.match(join_msg)[3..-1]
=> ["", "#channel", nil, nil]
```

Error will occur in lib/zircon/message.rb:43.
```
undefined method `empty?' for nil:NilClass (NoMethodError)
```
https://github.com/r7kamura/zircon/blob/master/lib/zircon/message.rb#L43

I don't understand perfectly Zircon::Message::Patterns::MESSAGE_PATTERN yet. (My knowledge of RegExp is a little...)
So I don't have self-confidence whether this approach is good...