DefaultVehicleHal: Add null callback checking

AIDL allow nullptr as callback argument.
Wihtout a nullptr callback variable checking, it will crash
the VHAL, if callback pass in is nullptr.

Test: android.hardware.automotive.vehicle@V1-default-service_fuzzer
Bug: 231661617
Change-Id: I9387391c595b608b435daa6e66134fedfccbbf7c
1 file changed