Set UserHandle to null instead of an empty ArrayBuffer for getAssertion.

https://www.w3.org/TR/webauthn/#ref-for-dom-authenticatorassertionresponse-userhandle

Bug: 847878
Change-Id: I8071207c15d3150a486133eb50cc7d00cbba5401
Reviewed-on: https://chromium-review.googlesource.com/1079200Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563073}

third_party/blink/renderer/modules/credentialmanager/credentials_container.cc

@@ -405,7 +405,7 @@ void OnGetAssertionComplete(
     DOMArrayBuffer* user_handle =
         credential->user_handle
             ? VectorToDOMArrayBuffer(std::move(*credential->user_handle))
-            : DOMArrayBuffer::Create(nullptr, 0);
+            : nullptr;
     AuthenticatorAssertionResponse* authenticator_response =
         AuthenticatorAssertionResponse::Create(client_data_buffer,
                                                authenticator_buffer,