Bug 32417: Allow NULL in biblionumber in $order->store

Bug 32417: Allow NULL in biblionumber in $order->store

If we are cancelling an order and call DelBiblio, we should
clear the biblionumber but not be crashing on it in ->store.

The absence of the clear prevented the crash. One bug sometimes
solves another one.

Test plan:
Create basket, order line.
Delete order with biblio record.
Verify result and logfile.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>