Fix videoalpha import from xml

VideoAlphaDescriptor inherits from EmptyDataRawDescriptor.
EmptyDataRawDescriptor is used for modules which might not have any
XML data, but require XmlDescriptor's functionality for
export/import. Without it, VideoAlphaModule would cause a crash on
export if any modules had an empty `self.data` field.

Don't use default video if HTML5 sources are given.
Merge pull request #603 from edx/alex/fix-videoalpha-importing-from-xml

Fix videoalpha import from xml