Bug 33567: Add a dbrev for installation with empty pref

Bug 33567: Add a dbrev for installation with empty pref

To keep current behavior, we can copy the removed fallback
into the syspref value in case someone might have cleared the pref.

Note: We are not restoring deleted prefs here; that is a data problem
outside the scope of this report. A regular installation should always
have this pref.

Test plan:
Run updatedatabase.pl
Bonus: Clear the pref Reference_NFL_Statuses and run again, verify that
the pref has changed.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>