disallow NULL on custom gradebook column columns

Change-Id: Ib6de3f7168a59d2be1d8663bb9901bcd102558a0
Reviewed-on: https://gerrit.instructure.com/28667
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>