Replace Null Returns with EmptyList in SubscriptionManager

Fix some call sites where a null might have been returned in
the specific edge case that the phone process is not available
and SubscriptionManager swallows the RemoteException.

This will also guard against any issues where the implementation
of SubscriptionManagerService erroneously returns a null when an
empty list could have been returned.

Bug: 245818227
Test: atest FrameworksTelephonyTest # (no functional change)
Test: Functional test (voice, messaging, data)
Change-Id: I2046567edbceaf9fef5f754a368b9aff8f18e6e8