allows for null selectedAssignment with AssignnmentMuter

fixes CNVS-10874

test plan:
- in screenreader gradebook
- navigate through assignments
- mute and unmute them
- set assignment back to null
- verify that you dont get errors in console
- verify that other srgb functionality works (prev/next navigation, grading)

Change-Id: I8021d0c120638149082ac8b0ee9b3ce703a122a3
Reviewed-on: https://gerrit.instructure.com/29519
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>