toExecutionContext() should return immediately if v8::Context is empty

Passed v8::Context can be empty in some cases, for example, when checking
an execution context to resolve/reject a promise after a window object
(v8 context) is gone.


BUG=n/a
TEST=n/a

Review URL: https://codereview.chromium.org/398223004

git-svn-id: svn://svn.chromium.org/blink/trunk@178355 bbb929c8-8fbe-4397-9dbb-9b2b20218538