Bug 31383: (QA follow-up) Do not return 0 in ->get

The last statement evaluated is the if( $content->count ).
If that evaluates to false, the false expression value is returned.
In this case that is 0.
We should just return undef or empty list (scalar/list context).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Koha/Template/Plugin/AdditionalContents.pm

@@ -53,6 +53,7 @@ sub get {
             blocktitle => $blocktitle
         };
     }
+    return;
 }
 
 sub get_opac_news_by_id {