fix(headlines-search): update condition for displaying topic description

- Change null check to isEmpty check for topic description
- Ensure proper handling of empty strings in topic description

Author: fulleni

lib/headlines-search/widgets/category_item_widget.dart

@@ -14,7 +14,7 @@ class TopicItemWidget extends StatelessWidget {
   Widget build(BuildContext context) {
     return ListTile(
       title: Text(topic.name),
-      subtitle: topic.description != null
+      subtitle: topic.description.isNotEmpty
           ? Text(
               topic.description,
               maxLines: 2,