Bug 28483: Remove warnings from Search.t

Bug 28483: Remove warnings from Search.t

There are too many warnings about uninitialized variables in Search.pm.
This patch deals with that, the same way it is dealt accross the file:
by setting an empty string when things are not defined.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Search.t
=> FAIL: Wow, too many warnings. Some related to this bug, some not.
2. Apply this patch
3. Repeat 1
=> SUCCESS: No more warnings!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>