Bug 25297: Consistent return value in K::A::Order->current_item_level_holds

Bug 25297: Consistent return value in K::A::Order->current_item_level_holds

This patch makes the return values for 'current_item_level_holds'
consistent: they will always be a Koha::Holds iterator.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Order.t
=> FAIL: It doesn't work as expected, cannot call ->count on undef, etc
3. Apply this patch
4. Repeat 2.
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>