DevTools: write empty stack as [] rather than null

This change unifies empty stack trace representation in trace events.

BUG=361045

Review URL: https://codereview.chromium.org/483373002

git-svn-id: svn://svn.chromium.org/blink/trunk@180570 bbb929c8-8fbe-4397-9dbb-9b2b20218538