Quizzes: Accessible move dialog moves empty groups

Fixes CNVS-16268

Test plan:
- In a quiz, create 2 question groups. At least one of these should be empty.
- Click the move handle on the left side of an empty question group.
- When the dialog opens, choose a position, and click 'Save'.
- Verify that all works properly.

Change-Id: I0b01c84419cdc00b1c507780a5de9833f7e488bc
Reviewed-on: https://gerrit.instructure.com/42927
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Finney <bfinney@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>