Bug 34520: Fix FK for item_groups in reserves for new installations

Bug 34520: Fix FK for item_groups in reserves for new installations

There was a discrepancy between the database update for reserves
and the kohastructure.sql definition. This makes sure that the
FK is always "ON DELETE SET NULL".

To test:
* Before applying this path
* sudo koha-mysql kohadev
* show create table reserves;

CONSTRAINT `reserves_ibfk_ig` FOREIGN KEY (`item_group_id`) REFERENCES `item_groups` (`item_group_id`) ON DELETE CASCADE ON UPDATE CASCADE

* Apply patch
* Run database update
* Check database again:

CONSTRAINT `reserves_ibfk_ig` FOREIGN KEY (`item_group_id`) REFERENCES `item_groups` (`item_group_id`) ON DELETE SET NULL ON UPDATE CASCADE

* reset_all (create a new database)
* Check database again:

CONSTRAINT `reserves_ibfk_ig` FOREIGN KEY (`item_group_id`) REFERENCES `item_groups` (`item_group_id`) ON DELETE SET NULL ON UPDATE CASCADE

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>