Add the return branch when ringtone position is POS_UNKNOWN

When the getCheckItem is POS_UNKNOWN, it is not the case we expected.
We return null for this case.

Test: manual start RingtonePickerActivity to use
EXTRA_RINGTONE_SHOW_DEFAULT false and EXTRA_RINGTONE_SHOW_SILENT false.
Check the returned EXTRA_RINGTONE_PICKED_URI, it should be null.

Bug: 165892509

Change-Id: Id473dad2e8672a65e5c1b0fee16a7d22e9bcb3e9