DevTools: Return empty ScriptValue instead of v8::Null from currentCallFrames.

This is a follow up clean-up.

R=yurys

Review URL: https://codereview.chromium.org/93603003

git-svn-id: svn://svn.chromium.org/blink/trunk@163781 bbb929c8-8fbe-4397-9dbb-9b2b20218538