require course assignment groups in a saner way

fixes CNVS-15252

test plan:
- create a new course and DO NOT visit the assignment index page
- go create a new ungraded discussion
- refresh the discussion show page a few times
- go to the assignment index page
- there should only be 1 assignment group (the default)
- repeat the process with an ungraded quiz

Change-Id: If6714bef05379a0f1253a0843d473695a1372456
Reviewed-on: https://gerrit.instructure.com/40709
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>