disallow nulls on data_exports table

Change-Id: I69aae29886ddf98fcdee3ce21ed3756aae078920
Reviewed-on: https://gerrit.instructure.com/32702
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>