[Settings] Avoid crash for VoIP account displayed as default voice

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