notificationDaemon: Try harder to find a matching app

Unlike the desktop-entry hint, the app name is not optional. That
doesn't mean that we'll be able to match it to a .desktop file,
but we can at least try if we fail to match on PID or desktop-entry.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1291


(cherry picked from commit b487846c)