fix(AlertReports): defaulting grace period to undefined (#27552)

(cherry picked from commit 4fce940)

Author: fisjac

superset-frontend/src/features/alerts/AlertReportModal.tsx

@@ -886,7 +886,7 @@ const AlertReportModal: FunctionComponent<AlertReportModalProps> = ({
 
     // Need to make sure grace period is not lower than TIMEOUT_MIN
     if (value === 0) {
-      updateAlertState(target.name, null);
+      updateAlertState(target.name, undefined);
     } else {
       updateAlertState(
         target.name,