refactor: Improve indexing notification handling and return values

- Enhanced error handling in the indexing process to differentiate between actual failures and cases where no new documents are processed.
- Updated notification messages to reflect the status accurately, including a message for when no new items are synced.
- Standardized return values across various indexer tasks to return `None` on success, simplifying logging and error management.

surfsense_backend/app/services/notification_service.py

@@ -358,8 +358,11 @@ class ConnectorIndexingNotificationHandler(BaseNotificationHandler):
             status = "failed"
         else:
             title = f"Ready: {connector_name}"
-            item_text = "item" if indexed_count == 1 else "items"
-            message = f"Now searchable! {indexed_count} {item_text} synced."
+            if indexed_count == 0:
+                message = "Already up to date! No new items to sync."
+            else:
+                item_text = "item" if indexed_count == 1 else "items"
+                message = f"Now searchable! {indexed_count} {item_text} synced."
             status = "completed"
 
         metadata_updates = {