Valera/fix empty captions panel no captions

Fix for bug BLD-277: "There is a white panel over a non-youtube video."

When a captions file was not specified, we do not show the captions panel. Also when an empty line was specified for the captions, we don't show the panel.

I the file was specified, but for some reason the browser wasn't able to retrieve it, a blank captions panel will be shown.

<i>Reviewers</i>
@polesye