Service Workers/Fetch: Fix Response constructor for empty/null

Source/modules/serviceworkers/Response.idl

 // Copyright 2014 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.
 
 // http://fetch.spec.whatwg.org/#response-class
 
 enum ResponseType { "basic", "cors", "default", "error", "opaque" };
 
 [
     // FIXME: Add ctors for FormData and URLSearchParams response bodies.
+    Constructor(),
     Constructor(USVString body, optional Dictionary responseInitDict),
-    Constructor(Blob? body, optional Dictionary responseInitDict),
+    Constructor(Blob body, optional Dictionary responseInitDict),
     Constructor(ArrayBuffer input, optional Dictionary requestInitDict),
     Constructor(ArrayBufferView input, optional Dictionary requestInitDict),
     ConstructorCallWith=ExecutionContext,
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     RaisesException=Constructor,
     ActiveDOMObject,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Response {
     // FIXME: Implement redirect().
     readonly attribute ResponseType type;
     readonly attribute USVString url;
     readonly attribute unsigned short status;
     readonly attribute ByteString statusText;
     readonly attribute Headers headers;
 
     Response clone();
 };
 
 Response implements Body;