Bug 13757: Make Koha::Patron::Modification->store del empty attrs

Bug 13757: Make Koha::Patron::Modification->store del empty attrs

This patch makes Koha::Patron::Modification->store delete the passed
attributes that contain empty values.

This is the way it currently works, as all opac-editable attributes are
presented to the end-user and they are allowed to delete them, and the
best way I found to represent the deletion on the modification request
is by setting it to the empty string. I chose this way because it is how
the staff interface handles it, so it is consistent.

To test:
- Apply this patch
- Run:
  $ prove t/db_dependent/Koha/Patron/Modifications.t
=> SUCCESS: This time tests pass!
- Verify comment #70 on the bug is fixed now
- Sign off :-D

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>