Check for nullptr on av_source dumpsys am: 5d3c67df am: 1244357d am: efa01915

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2084171



Change-Id: I27459950e8726a6876355eaf5d4b32dfc20a09da
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>