Streamline v8NullWithCheck.

It's unnecessary to create a Handle and then a second one right away.

This won't make a performance difference with the current master, but will make
a difference once https://codereview.chromium.org/19799002/ gets relanded.

BUG=264056
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/20681002

git-svn-id: svn://svn.chromium.org/blink/trunk@154987 bbb929c8-8fbe-4397-9dbb-9b2b20218538