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 
Now modulestore is present in XML course but `course.static_asset_path` is not set but `asset_path` is set in `course.data_dir`.