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>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

Koha/Object.pm

@@ -709,7 +709,8 @@ sub attributes_from_api {
         }
         elsif ( _date_or_datetime_column_type( $columns_info->{$koha_field_name}->{data_type} ) ) {
             try {
-                $value = dt_from_string($value, 'rfc3339');
+                $value = dt_from_string($value, 'rfc3339')
+                    if defined $value;
             }
             catch {
                 Koha::Exceptions::BadParameter->throw( parameter => $key );