Return null when no promise/reason is given in PromiseRejectionEvent

Source/core/events/PromiseRejectionEvent.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://github.com/domenic/unhandled-rejections-browser-spec
 
 [
     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;
 };