fix migrations for someone stuck with a real default shard but no settings

Change-Id: I94fe2900dece2e6422e69d37595ddb8b03317318
Reviewed-on: https://gerrit.instructure.com/37766
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>