Non-null constraints for quiz_submission_events

Closes CNVS-16408

TEST PLAN
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N/A

Change-Id: I798cda1bd10fd4bc7c19253bbaefd7927efe56c0
Reviewed-on: https://gerrit.instructure.com/43205
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>