Bug 22690: (QA follow-up) Make bib-level hold object actually bib-level

Bug 22690: (QA follow-up) Make bib-level hold object actually bib-level

We need to pass undef itemnumber to build_object() to actually have a
hold without an item tied to it. Otherwise build_object() will create
automatically an item for us (thus making it an item-level hold)

To test:
 $ prove t/db_dependent/Koha/Item.t

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>