Fix word cloud modules not exporting correctly.
More general solution to word cloud export problems.
Add descriptor for modules with empty XML data.

This allows a more general approach to modules such as word_cloud and
video which have no XML data (it's all store in metadata), but use
XmlDescriptor for backwards compatibility. They now generate an empty
tag on export, and clear out empty tags on import.

Also a small change to the video module as a result -- if it's asked
to parse empty XML data, it won't try to parse anything.
Merge pull request #492 from edx/peter-fogg/fix-stud-146

Fix word cloud modules not exporting correctly.