Make assignment overrides editable without root account ids
Make assignment overrides editable without root account ids

fixes CNVS-49603

test plan:
- Create an assignment with an override
- In rails console, make the root_account_id attribute of the override nil
- Attempt to edit the assignment date/assign-to (as described in the jira)
- It should work

Change-Id: Ib4c8e94ab68d181db79b338117e8af9bc936ad3e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/241950
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>
QA-Review: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>
Product-Review: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>

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