fix MediaObject.refresh_media_files job for assessment questions

don't puke trying to find folders for things that don't have folders

Change-Id: Ie2291ecb4a8bf63b3c550b5c35b543bdb5257ddd
Reviewed-on: https://gerrit.instructure.com/27943
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>