bindings: Use V8SetReturnValueNull() in the custom bindings.

Small simplification that should not have any user-visible effects.

Change-Id: I93764d578de2e6e84c013f5e8a44392d0938835b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1941887
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719954}

third_party/blink/renderer/bindings/core/v8/custom/v8_promise_rejection_event_custom.cc

@@ -35,7 +35,7 @@ void V8PromiseRejectionEvent::PromiseAttributeGetterCustom(
 
   ScriptPromise promise = event->promise(ScriptState::Current(isolate));
   if (promise.IsEmpty()) {
-    V8SetReturnValue(info, v8::Null(isolate));
+    V8SetReturnValueNull(info);
     return;
   }
 

third_party/blink/renderer/bindings/core/v8/custom/v8_xml_http_request_custom.cc

@@ -79,7 +79,7 @@ void V8XMLHttpRequest::ResponseAttributeGetterCustom(
       v8::Local<v8::String> json_source =
           xml_http_request->ResponseJSONSource();
       if (json_source.IsEmpty()) {
-        V8SetReturnValue(info, v8::Null(isolate));
+        V8SetReturnValueNull(info);
         return;
       }
 
@@ -90,7 +90,7 @@ void V8XMLHttpRequest::ResponseAttributeGetterCustom(
                          ToCoreString(json_source), exception_state);
       if (exception_state.HadException()) {
         exception_state.ClearException();
-        V8SetReturnValue(info, v8::Null(isolate));
+        V8SetReturnValueNull(info);
       } else {
         V8SetReturnValue(info, json);
       }