renders correct alert for speedgrader with empty groups
renders correct alert for speedgrader with empty groups

fixes CNVS-8267

test plan
- create a new set of groups
- leave the groups as empty
- create a new group assignment
- go to speedgrader for the new assignment
- verify that the correct error message is an alert pop up
- error message should prompt user to add students to groups

Change-Id: Ib7543a4d337e579b8e35630af2ab370a69ef7187
Reviewed-on: https://gerrit.instructure.com/26410
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Cameron Sutter <853bcc0cb8f5834b7805e8eb9a762054be43c731@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Liz Abinante <ce3305c9be70e1a5a9ff49df19e1a5b24d5a47f8@instructure.com>

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