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>

Koha/Cache.pm

@@ -271,10 +271,11 @@ sub set_in_cache {
     # Deep copy if it's not a scalar and unsafe is not passed
     $value = dclone( $value ) if ref($value) and not $unsafe;
 
-    # Set in L1 cache
+    # Set in L1 cache; exit if we are caching an undef
     $L1_cache{ $key } = $value;
+    return if !defined $value;
 
-    # We consider an expiry of 0 to be inifinite
+    # We consider an expiry of 0 to be infinite
     if ( $expiry ) {
         return $set_sub
           ? $set_sub->( $key, $value, $expiry )