Bug 29452: remove unnecessary warns in the logs

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/SIP/Sip.pm

@@ -99,7 +99,7 @@ sub maybe_add {
         return q{} if first { $_ eq $fid } @fields;
     }
 
-    if ( $fid eq FID_SCREEN_MSG && $server->{account}->{screen_msg_regex} ) {
+    if ( $fid eq FID_SCREEN_MSG && $server->{account}->{screen_msg_regex} && defined($value)) {
         foreach my $regex (
             ref $server->{account}->{screen_msg_regex} eq "ARRAY"
             ? @{ $server->{account}->{screen_msg_regex} }