Bug 29157: Date and Datetime attributes cannot be set NULL

Bug 29157: Date and Datetime attributes cannot be set NULL

This patch makes Koha::Object->attribute_from_api allow setting
attributes the undef value. The original implementation passed the value
directly to dt_from_string, which made the attribute be set the current
date.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Object.t \
           t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail! Our code is buggy!
3. Apply this patch
4. Repeat 2
=> SUCCESS! Fix fixed this thing!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Eric Phetteplace <phette23@gmail.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>