Null check IMS calls in shouldDisconnectActiveCallOnAnswer

Adds a check to shouldDisconnectActiveCallOnAnswer to
ensure that the active IMS call and incoming IMS call
are non-null. This can happen in cases when the
ImsPhoneCallTracker moves into a bad state.

Bug: 36273870
Test: Telephony Unit Tests
Change-Id: Ia31bb8cc6e425b63f836e8244bb3919d10c50731