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 
XML courses use the `ImportSystem`, which does store the modulestore (https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/modulestore/xml.py#L71). Is this failing only in tests, or is it actually an issue in the LMS when it's live?