fix appointment group callbacks for rails 3 specs

NullStore returns false in rails 3

Change-Id: Id599bae31af71ece4faaacd8b626d6af20be1204
Reviewed-on: https://gerrit.instructure.com/28460
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>