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 
Looking at git blame leads me to believe that changed with https://github.com/edx/edx-platform/commit/a9213509c38d0177b5d8bd7b6056901f39603c70 so I think this probably should have changed here instead of adding additional conditional logic. I think that test should be updated to exercise that new else as well