Bug 14717: Invalid dates in debarred column

Bug 14717: Invalid dates in debarred column

To test
1/ Import a patron using the patron import tool, make sure they have
no debarred column in the file
2/ Check the database, notice the debarred column is 0000-00-00
3/ For bonus points, checkout an item to that borrower, then check it in
notice Koha errors
4/ Apply patch
5/ Import a new patron
6/ Notice column is now NULL and that checkins work

Signed-off-by: Eugene Espinoza <eugenegf@yahoo.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>