When displaying VoIP account in Default voice account UI within
mobile network configuration, some null pointer checking need to
applied due to this is no longer a real account bind to SIM card.
Bug: 157334667
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=DefaultSubscriptionControllerTest
Change-Id: I3927362676c867ac245f16e1d00ea953b21ec1d4