Fix transcripts in XML course when no static_asset_path is set.

Case where static_asset_path is not set in XML course.

In such case use data_dir.
Issue: https://openedx.atlassian.net/browse/BLD-67

@cpennington @carsongee 
I find it strange that the else clause I wrote for XMLModuleStore isn't getting touched here.  I even had a test for it at https://github.com/edx/edx-platform/blob/oleg/xml-course-transcripts-fix/lms/djangoapps/courseware/tests/test_video_handlers.py#L440 and had tested it by hand as well.  If my modulestore runtime assumption is wrong, you may as well switch to just using the one else clause and removing this one