Bug 13757: (regression tests) Empty attributes should delete existing

Bug 13757: (regression tests) Empty attributes should delete existing

This patch introduces tests for the required functionality.

To test:
- Run:
  $ prove t/db_dependent/Koha/Patron/Modifications.t
=> FAIL: The current code doesn't work like that

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
https://bugs.koha-community.org/show_bug.cgi?id=13737

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>