Update api/services/app_service.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

api/services/app_service.py

@@ -230,8 +230,8 @@ class AppService:
         app.name = args.get("name", "")
         app.description = args.get("description", "")
         app.icon_type = args.get("icon_type", "emoji")
-        app.icon = args.get("icon", "")
-        app.icon_background = args.get("icon_background", "")
+        app.icon = args.get("icon", None)
+        app.icon_background = args.get("icon_background", None)
         app.use_icon_as_answer_icon = args.get("use_icon_as_answer_icon", False)
         app.max_active_requests = args.get("max_active_requests")
         app.updated_by = current_user.id