fix MediaObject.add_media_files failures

don't look for sis pseudonyms when processing media files that belong
to things that don't have root accounts

Change-Id: Ia77a9eb2755d11475fba9391a8e27a6126c669c4
Reviewed-on: https://gerrit.instructure.com/27942
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>