Fix Settings crash when setting a null ringtone

Ringtone picker may callback a null ringtone Uri
if users select None.

This change pass null ringtone Uri to RingtoneManager
and return.

Bug: 232502532
Bug: 221041256
Test: maunal
      Settings - Sound & Vibration -> Phone ringtone
      -> My Sounds -> None
Change-Id: I044b680871472a3c272f6264c4ef272df542112e