account notifications by role bugfix

specs were not catching the case where the
notification was created by site admin intended for
account roles on other accounts; as a result, a
NoMethodError slipped through. This commit fixes
the error and adds specs for this case

Change-Id: Ie3b3659aacae277a30a6e59ee0e0833504c0cd67
Reviewed-on: https://gerrit.instructure.com/31840
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>