When generating the action label, we expect both directions of these
features to have consistent settings (either both get a keycombo, or
they don't) or these just return NULL altogether.
Since one of the directions has an action associated, this is
misleading, so be more lenient at the time of generating the action
label.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2001>