Bug 22844: Make the patron's attribute mandatory at the OPAC

Bug 22844: Make the patron's attribute mandatory at the OPAC

Test plan:
- Set 1+ patron's attribute(s) mandatory
- Use the self-registration feature and confirm that you cannot selfreg
if the attribute has no value (or empty string)
- Same with the modification form (logged in)

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>