fix missing block when there is no failure (#18775)

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

utils/notification_service_doc_tests.py

@@ -167,7 +167,7 @@ def payload(self) -> str:
         if self.n_failures > 0:
             blocks.extend([self.category_failures])
 
-        if self.no_failures == 0:
+        if self.n_failures == 0:
             blocks.append(self.no_failures)
 
         return json.dumps(blocks)