don't empty pane_current_path if new one is empty
don't empty pane_current_path if new one is empty

sometimes even repeated polling for a new pane path over 1,
3, 5 seconds won't return anything in some instances.

In older cases, we'd remove the pane_current_path from the pane's
_info, now we'll keep the last non-empty result libtmux found until
a new one fills it.

tony/libtmux