Bug 35414: Silence warn related to number_of_copies

Bug 35414: Silence warn related to number_of_copies

To test in ktd:
- Tail the logs with "sudo tail -f /var/log/koha/kohadev/plack*.log"
- Find a record in the staff client
- Add an item to the record
- Observe "[WARN] Argument "" isn't numeric in subroutine entry ..."
  in /var/log/koha/kohadev/plack-intranet-error.log
- Apply this patch
- restart_all
- Tail the logs again
- Add another item
- Observe the absence of the earlier warning

Thanks to Jonathan Druart for suggesting this particular solution!

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>