Bug 35190: Set default NULL for authorised_value_category in additional_fields table

Bug 35190: Set default NULL for authorised_value_category in additional_fields table

To test:
1. Start KTD without this patch
2. In the Administration module, create an Additional Field with
   Authorized Value set to None
3. Create another Additional Field with Authorized Value set to an
   existing authorized value
4. Apply patch and updatedatabase
5. Access the database in the command line with koha-mysql kohadev
6. Select all rows from additional_fields
7. Confirm that the two Additional Fields from earlier have values of
   NULL and the correct authorized value, respectively, in their
   authorised_value_category fields.
8. reset_all to test new install
9. Access the database in the command line
10. Show the table structure for additional_fields and confirm that
    default for authorised_value_category is 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>
(cherry picked from commit 40414f9c694f3d044f81662c6d9df9c6ca19e2fc)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>