Bug 14651: (regression test) fallback to bib-level if itype is undef

Bug 14651: (regression test) fallback to bib-level if itype is undef

Koha::Item->effective_itemtype should fallback to biblio-level itemtype
even if item-level item types are set, in the case the item has no itemtype
set (bad migration, bad old code).

To test:
- Run
  $ prove t/db_dependent/Items.t
=> FAIL: Koha::Item->effective_itemtype doesn't work properly

Edit: Added a test for a warning when falling back as per QA request
and because it made a lot of sense :-D

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>