Return null when no promise/reason is given in PromiseRejectionEvent

Source/core/events/PromiseRejectionEvent.idl

Index: Source/core/events/PromiseRejectionEvent.idl
diff --git a/Source/core/events/PromiseRejectionEvent.idl b/Source/core/events/PromiseRejectionEvent.idl
index a1a04a3fe8d0e1befb6cb1f7a28dff9f1d82adbb..ccae17a06dd8dcc8e7cf494132b57a90a56f9008 100644
--- a/Source/core/events/PromiseRejectionEvent.idl
+++ b/Source/core/events/PromiseRejectionEvent.idl
@@ -7,8 +7,9 @@
 [
     RuntimeEnabled=PromiseRejectionEvent,
     Constructor(DOMString type, optional PromiseRejectionEventInit eventInitDict),
+    ConstructorCallWith=ScriptState,
     Exposed=(Window,Worker,ServiceWorker),
 ] interface PromiseRejectionEvent : Event {
-    [CallWith=ScriptState] readonly attribute Promise<any> promise;
+    [Custom=Getter] readonly attribute Promise<any>? promise;
     [CallWith=ScriptState] readonly attribute any reason;
 };