Right now we set a subtitle of "" when the port is invalid. This
leads to the "Port" title getting center aligned vertically.
We also keep the port sensitive even when it's empty and there is
little value in copying it.
This commit changes the subtitle to " " so it serves as a
placeholder to keep the "Port" title top aligned. This commit
also makes the port insensitive in that case so the user never
copies the placeholder space.
Closes: https://gitlab.gnome.org/GNOME/gnome-control-center/-/issues/2934
857265c250
