Bug 16809: Follow-up for scalarizing biblionumber

Bug 16809: Follow-up for scalarizing biblionumber

Still resolves another multi_param warning.

Test plan:
Look at your logs before and after this patch when saving a biblio
record (you may have to start plack again).
If your biblionumber is mapped to 999c, you should no longer have a warn
about line 2563 (disclaimer: line numbers are subject to change).

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: Kyle M Hall <kyle@bywatersolutions.com>