Fix for bug Blades/BLD-152

Original problem (including description from https://edx-wiki.atlassian.net/browse/BLD-152 ):
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If I remove the youtube attribute of the videoalpha player, I get an error from youtube rather than failing over to the other videos that I have provided.

<videoalpha show_captions="true" sub="name_of_file" >
<source src="https://s3.amazonaws.com/edx-course-videos/edx-intro/edX-FA12-cware-1_100.mp4"/>
<source src="https://s3.amazonaws.com/edx-course-videos/edx-intro/edX-FA12-cware-1_100.webm"/>
<source src="https://s3.amazonaws.com/edx-course-videos/edx-intro/edX-FA12-cware-1_100.ogv"/>
</videoalpha>
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The fix is basically making sure that either an empty string is passed or a correctly formated string of YouTube IDs.

@rocha Please review.