Fixed NPE in TelephonyBackupAgent am: 52ca68c81e am: f90a9b1124 am: 2a1dbbf640

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/providers/TelephonyProvider/+/10843582

Change-Id: I6990eecf385c78ed254f5bab9733ba9c16b7e3bc