Ignore null binding on service died

When a service dies, sometimes it will send onNullBinding
after onServiceDied. This should be ingored or else we will
permanently unbind until the user reboots.

Bug: 161139847
Test: atest FrameworksTelephonyTests:ImsServiceControllerTest
Change-Id: I1b3ae578ad318d176e8fad86c4b768aebe9c893f