Bug 32151: Resolve uninitialized warn in numeric ne in C4::Ris

We should not write:
    if ($kwfield != undef) {

Test plan:
Pick record with UTF8 chars.
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Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

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

C4/Ris.pm

@@ -831,7 +831,7 @@ sub get_keywords {
 
     ## loop over all 6XX fields
     foreach my $kwfield (@keywords) {
-        if ($kwfield != undef) {
+        if( defined $kwfield ) {
             ## authornames get special treatment
             if ($fieldname eq "600") {
                 my $val = normalize_author($kwfield->subfield('a'), $kwfield->subfield('b'), $kwfield->subfield('c'), $kwfield->indicator('1'));