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

- Change null check to empty string check for source description
- Improve accuracy of displayed information in SourceItemWidget

Author: fulleni

lib/headlines-search/widgets/source_item_widget.dart

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