Bug 32478: (QA follow-up) Keep current hashref behavior

Bug 32478: (QA follow-up) Keep current hashref behavior

Prevent a crash on wrong contents for ItemsDeniedRenewal pref
as we did before.
Note: Could be a provisional measure (no band aid to repeat anywhere)
until we resolve this in preferences.pl.

Test plan:
Without this patch:
Change ItemsDeniedRenewal to 'nonsense'
Run perl -MKoha::Items -e'Koha::Items->find(X)->is_denied_renewal; print "OK\n"'
=> Replace X by a valid itemnumber
Crashes with: Can't use string ("nonsense") as a HASH ref ... No OK print.

Apply this patch
Run perl -MKoha::Items -e'Koha::Items->find(X)->is_denied_renewal; print "OK\n"'
=> Replace X by a valid itemnumber
Warns only with: Hashref expected for ItemsDeniedRenewal. You got OK.
Clear ItemsDeniedRenewal
Try again. No warning anymore.
Run t/Context.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>