work around weird mysql behavior with text column defaults

refs CNVS-7032

Change-Id: Ia6474a8ae39e78b858a993993852139ec9f0c624
Reviewed-on: https://gerrit.instructure.com/28795
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>