Bug 26367: Prevent warn about undefined values when record level hold has an itemtype

Bug 26367: Prevent warn about undefined values when record level hold has an itemtype

To test:
 1 - set AllowHoldItemTypeSelection to 'Allow'
 2 - Find a patron from Library A
 3 - Find a record with an item from Library A
 4 - Place a title level hold with itemtype specified for a delivery at Library A for patron and record above
 5 - perl misc/cronjobs/holds/build_holds_queue.pl
 6 - There are warns:
 Use of uninitialized value in hash element at /kohadevbox/koha/C4/HoldsQueue.pm line 523.
 Use of uninitialized value in string eq at /kohadevbox/koha/C4/HoldsQueue.pm line 523
 7 - Apply this patch
 8 - perl misc/cronjobs/holds/build_holds_queue.pl
 9 - No more warns!
10 - prove -v t/db_dependent/HoldsQueue.t
11 - All tests pass!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>