Tracing fix: don't try to retrieve file name from empty scripts.

After this, we don't get trace events for running empty scripts (e.g., scripts
where compilation has failed) but that's most probably okay, since those trace
events would be meaningless anyway (it's an early return).

R=jochen@chromium.org
BUG=387948

Review URL: https://codereview.chromium.org/334933004

git-svn-id: svn://svn.chromium.org/blink/trunk@176829 bbb929c8-8fbe-4397-9dbb-9b2b20218538