spec: fix ordering of nulls for mysql in conversations spec

refs CNVS-7032

Change-Id: I46f934990a89bbabf8348feca67969d6a2c24d29
Reviewed-on: https://gerrit.instructure.com/28609
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>