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

LayoutTests/http/tests/serviceworker/resources/fetch-event-test-worker.js

 function handleString(event) {
   event.respondWith(new Response('Test string'));
 }
 
 function handleBlob(event) {
   event.respondWith(new Response(new Blob(['Test blob'])));
 }
 
 function handleReferrer(event) {
   event.respondWith(new Response(new Blob(
     ['Referrer: ' + event.request.referrer])));
 }
 
 function handleNullBody(event) {
-  event.respondWith(new Response(null));
+  event.respondWith(new Response());
 }
 
 function handleFetch(event) {
   event.respondWith(fetch('other.html'));
 }
 
 function handleFormPost(event) {
   event.respondWith(new Promise(function(resolve) {
       event.request.text()
         .then(function(result) {
@@ skipping 36 matching lines @@
       }
     }
 
     if (handler) {
       handler.fn(event);
     } else {
       event.respondWith(new Response(new Blob(
         ['Service Worker got an unexpected request: ' + url])));
     }
   });