Ensure that onCellInfo is always NonNull

During phone creation and phone count changes,
it was possible to get a null value for onCellInfo().

Since the API is documented as NonNull, always return
a non-null value, forcing an empty list in case there
is still a null lurking somewhere else.

Bug: 237964039
Test: atest TelephonyRegistryTest#testCellInfoChanged
Change-Id: I3b46c211f5542a39187b9d3b137f8483c44ccd39