Bug 34051: Cache empty hashref for non-existent authorised values

Bug 34051: Cache empty hashref for non-existent authorised values

When fetching authorised value descriptions, we need to handle
scenarios where a value doesn't exist in the cache and doesn't
exist in the database. In this situation, we return an empty
hashref, and this patch makes us cache this empty hashref.

This is important because otherwise the function
Koha::AuthorisedValues->get_description_by_koha_field will
do a database call every time it encounters the same value that
doesn't exist in the authorised values table.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>