Add null check for magnificationSettingsController

Since the DisplayIdIndexSupplier#get could return a null controller, we
need to check if the controller is null before performing the next
steps.

Bug: 378043155
Test: atest IMagnificationConnectionTest
Flag: EXEMPT bugfix
Change-Id: I494e6f0b69c6ef47a71920d5e5e3e115b6c6d2c4