Bug 24529: Silence warnings for title level holds

Bug 24529: Silence warnings for title level holds

This patch simply adds a test for itemnumber being defined in the
reserve before comparing it to silence the warning.

Test plan:
1) Prior to patch, run prove t/db_dependant/Circulation.t and note lots
   of `Use of uninitialized value in numeric eq (==) at
   /kohadevbox/koha/C4/Reserves.pm line 790` warnings.
2) Apply the patch, re-run the test and note the tests still pass but
   the warnings are now gone.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>