[ServiceWorker] Make fetch() method better conformance with the spec.

Source/modules/serviceworkers/Response.cpp

Index: Source/modules/serviceworkers/Response.cpp
diff --git a/Source/modules/serviceworkers/Response.cpp b/Source/modules/serviceworkers/Response.cpp
index ffc8f1e9d7cb1242e4f1b6890236c34bc53af24a..77846007b38001952d357c5f38454c8848ff35c5 100644
--- a/Source/modules/serviceworkers/Response.cpp
+++ b/Source/modules/serviceworkers/Response.cpp
@@ -153,6 +153,8 @@ PassRefPtrWillBeRawPtr<Headers> Response::headers() const
 
 PassRefPtrWillBeRawPtr<FetchBodyStream> Response::body(ExecutionContext* context)
 {
+    if (!m_response->blobDataHandle())
+        return nullptr;
     if (!m_fetchBodyStream)
         m_fetchBodyStream = FetchBodyStream::create(context, m_response->blobDataHandle());
     return m_fetchBodyStream;