avoid extra submissions query in assignment_json

refs CNVS-16051

Test plan:
  * make two assignments (and publish them).
  * make submissions to one of the assignments.
  * on the assignments index page, you should be able to unpublish the
    submisssion-less assignment with the cloud icon.
  * you should not be able to unpublish the other assignment.
  * a tooltip will explain why

Change-Id: I707f6939af43b4d9a2ac1550c26c81f137e2ae7c
Reviewed-on: https://gerrit.instructure.com/42458
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>