Bug 20144: (follow-up) Adjust /patrons endpoint

Bug 20144: (follow-up) Adjust /patrons endpoint

Fields 'address' and 'city' have been maybe nullable by this bug. This
patch makes them nullable on the API as well.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail (randomly) when address or city are set to null by
         TestBuilder.
- Apply this patch
- Run:
 k$ prove t/db_dependent/api/v1/patrons.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>