list_windows: We may be filtering out `window_name: ''` and `''` values

[tony]

#444 it seems the test will always pass, even before the change that, when verified by hand, permits window_name: '' to work (or else it'd default to zsh or whatever the shell name was for the window)

See if our filtering is negating '' as a falsey value. We want to keep those, if they exist.

Fulll context: https://github.com/tmux-python/libtmux/pull/444#discussion_r980591231