optimize students with no submissions report

fixes CNVS-10782

test plan
 - run students with no submissions report
 - it should run much faster

Change-Id: Ib57bac057e61a9ed7a0b2656b3f80c6efe134aa6
Reviewed-on: https://gerrit.instructure.com/29350
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>