Bug 20696: Fix a few ugly "eq undef" comparisons in Search.pm

Bug 20696: Fix a few ugly "eq undef" comparisons in Search.pm

Comparisons like $a eq undef should normally raise a warning like:
    Use of uninitialized value in string eq at ...
But unfortunately we still suppress warnings here and there.

Test plan:
[1] Just read this patch and confirm the small changes.
[2] Git grep on "eq undef" and do not find other occurrences.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Passes test plan and QA tools. Searching works correctly.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 66f5b793ed8e52372466122263cac0ddd84dea63)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>