Add a null pointer check for getSubscriptionsInGroup

Even though #getSubscriptionsInGroup is marked @NonNull, when the
Phone process crashes and this method is called from the system
process, it will return null.

Bug: 295339956
Test: atest CtsVcnTestCases, FrameworksVcnTests
Change-Id: I9f827f9e47efef08e9905c9cc6947df84651383b