Bug 13640: (QA follow-up) Allow NULL values for default_branch_item_rules.holdallowed

Bug 13640: (QA follow-up) Allow NULL values for default_branch_item_rules.holdallowed

If a rule has not been defined for an itemtype or you submit a biblio level
hold, you cannot filter on holdallowed<>0.

Test plan:
Test with a biblio level hold. Have two items; one item should be blocked
with a policy.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>