ImsReasonInfo set an initial value and added a null check

the last ImsReasonInfo value was kept when callback was registered without initial set.
To prevent @NonNull violations, ImsReasonInfoi set an initial value and added a null check.

Bug: 267671502
Test: atest CtsTelephonyTestCases: PhoneStateListenerTest
Test: atest CtsTelephonyTestCases: TelephonyCallbackTest
Test: atest TelephonyRegistryTest
Test: manual

Change-Id: I884f4ce12fded530a1062bc7c11999334118c63d