Bug 16412: Cache undef in L1 only

Bug 16412: Cache undef in L1 only

No need to cache undef in L2.
Prevent numerous warnings like:
Use of uninitialized value in subroutine entry at /usr/share/koha/masterclone/Koha/Cache.pm line 284

Test plan:
Run t/Cache.t
Run t/db_dependent/sysprefs.t

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch: remove trailing space char and replace 'you' with 'we' to
match existing wordings.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>