Bug 31535: Fix warning - uninitialized value $mode in string ne (addbiblio.pl)

Line number varies across versions. (Should be L887 now.)
Resolves:
    Use of uninitialized value $mode in string ne at /usr/share/koha/cataloguing/addbiblio.pl line 845.

No test plan here. This is trivial. Read the patch.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

cataloguing/addbiblio.pl

@@ -719,7 +719,7 @@ my $parentbiblio  = $input->param('parentbiblionumber');
 my $breedingid    = $input->param('breedingid');
 my $z3950         = $input->param('z3950');
 my $op            = $input->param('op') // q{};
-my $mode          = $input->param('mode');
+my $mode          = $input->param('mode') // q{};
 my $frameworkcode = $input->param('frameworkcode');
 my $redirect      = $input->param('redirect');
 my $searchid      = $input->param('searchid') // "";