Fix issue when no pane are specified

I don't use *panes* in my tmux configuration, each of my *windows* only have the main *pane*. This was creating an issue where `self.panes` was `nil`.

Teamocil was trying to run the `splits` method which doesn't seem to exist. I've replaced the assignment with an empty Array.

Not sure it's the correct way to fix it but it works perfectly for my situation.