I've just had a look into tmux's code, and the option defaults to `off`:
http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/options-table.c#l138

So the `-z` check in the module is irrelevant as there's always a value, set either by the user or tmux itself. (It's very nice of tmux to make the default explicit for us, by the way!)

The code of the module must take this into account, I'll update accordingly.

Thanks for pointing that out.