fix compile error

spec.bs

@@ -1246,7 +1246,7 @@ Several APIs specific to fenced frames are defined on the {{Fence}} interface.
   [Exposed=Window]
   interface Fence {
       undefined reportEvent(ReportEventType event);
-      undefined setReportEventDataForAutomaticBeacons(FenceEvent event);
+      undefined setReportEventDataForAutomaticBeacons(optional FenceEvent event);
       sequence<FencedFrameConfig> getNestedConfigs();
   };
 </pre>