disallow null on custom_gradebook_columns.teacher_notes

Change-Id: I51ab4f95ba2a1ba75515c117735575d1b28225c6
Reviewed-on: https://gerrit.instructure.com/28897
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>