config: Webserver Home Page form value defaults to "Apache Default" when not set on disk

I noticed this with a new FreedomBox on Debian Bullseye / Bookworm.

When I go to the Configuration page, the Webserver Home Page value appears to be set to "Apache Default", even though this is not really the case. (/etc/apache2/conf-enabled/freedombox-apache-homepage.conf is not written to disk yet.) Then if I click Update setup, I get a message that the Webserver Home Page has been changed.

If there is no setting on disk, perhaps we can display the current setting as blank, or use "FreedomBox Service (Plinth)" as the default.