[Pandora] Return RemoteNotFound when device name is null

BluetoothDevice::getName() returns null when there is no record cached,
but setting a null value in proto raises errors, so we should simply
returns not found.

Bug: 245578454
Test: atest -v avatar
Ignore-AOSP-First: Cherry-picked from AOSP
Merged-In: Ia409b61491f4f13982a07db9c11667aabb246bb1
Change-Id: Ia409b61491f4f13982a07db9c11667aabb246bb1