Add null check in vibration util methods

To avoid an exception when given ringtone uri
is null, add null check in hasVibration method.

Also add a check in getVibrationUri method to
avoid any potential exception in the future.

Flag: EXEMPT bugfix
Fix: 374858125
Test: atest RingtoneManagerTest
Test: atest RingtoneTest
Test: atest RingerTest
Change-Id: I51b28dcd349ac040273168aa6bac1427f20d7d95