fix ChangeEnrollmentsRoleIdNull

test plan: fixes migrations with lingering StudentViewEnrollments
with null role_id

fixes CNVS-17644

Change-Id: Ic75dee9a73447afdd21aabb0ef9732f9dcd74d30
Reviewed-on: https://gerrit.instructure.com/46239
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>