Bug 15087: hardduedate should not be inserted as 0000-00-00 but NULL

Bug 15087: hardduedate should not be inserted as 0000-00-00 but NULL

This has been introduced by bug 14949, recently pushed.
So I don't think we need to update the DB, only devs will face this
problem.

The hardduedate was inserted as 0000-00-00 because was en empty string.
To be inserted as NULL, the value should be undef.

Test plan:
0/ Don't apply this patch
1/ Edit a circ rules and don't fill the hard due date
2/ Look at the DB, the value is 0000-00-00
3/ Apply this patch and confirm that the value is now NULL

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Confirmed
+--------------+----------+-------------+
| categorycode | itemtype | hardduedate |
+--------------+----------+-------------+
| *            | *        | NULL        |
+--------------+----------+-------------+

No koha-qa errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>