Bug 35190: Adjust UI handling to avoid inserting an empty string

Bug 35190: Adjust UI handling to avoid inserting an empty string

To test:
1. In the Administration module, create an Additional Field that is
   not linked to an authorized value
2. Create another Additional Field that is linked to an authorized
   value
3. Access the database in the command line (koha-mysql kohadev)
4. Select all from additional_fields
5. Confirm that each field correctly lists either the appropriate
   authorized value, or NULL

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>