Fix a Null retval in SM#getSubscriptionsInGroup

In the event that the SubscriptionService can't be
reached, system callers receive a null return value
instead of receiving the documented NonNull result.

Simple fix to ensure that the return value is an empty
list.

Bug: 296125268
Test: atest FrameworksTelephonyTests
Test: MO/MT voice call, MO/MT sms, data
Change-Id: I21f536e8739c1fb27caeac3266e964eb64c56a3e