Bug 13075: (followup) remove remaining warnings

Bug 13075: (followup) remove remaining warnings

There's no point creating a MARC record with undef subfields
for testing holds. This patch avoids that so no warnings are shown.

To test:
- Run
  $ prove t/db_dependent/Holds.t
=> FAIL: verify several warnings show
- Apply the patch
- Re-run
=> SUCCESS: no warnings showed.
- Sign off :-D

Regards

NOTE: Not noticable under Ubuntu 12.04 LTS, but verifiable under
      Debian Wheezy.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>