toV8Value should return a JS Null value when a null pointer is given.

It is helpful to return a JS Null Value from V8ValueTraits<T>::toV8Value
when a null pointer is given.

BUG=390894

Review URL: https://codereview.chromium.org/368933004

git-svn-id: svn://svn.chromium.org/blink/trunk@178143 bbb929c8-8fbe-4397-9dbb-9b2b20218538