Bug 31535: Fix warning - uninitialized value in string ne (MARCdetail.pl)

Bug 31535: Fix warning - uninitialized value in string ne (MARCdetail.pl)

Spotted at line 183 (older Koha version).

Test plan:
You need a NULL in the tab column of marc_subfield_structure. Hit this
page and check the logs.
This might convince you easier:
  my $tabloop=0; my $x; print "Empty string - True\n" if ( $x // q{} ) ne $tabloop;
  print "Zero - True\n" if ( $x // '0' ) ne $tabloop;
You should only see: Empty string - True proving that defaulting to '0'
would be a game changer.

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>