Camera: Vts: update check for setCallback

A Rust HAL cannot return a service specific error when a nullptr is
passed to setCallback, since libbinder_rs checks for nullptr and returns
EX_NULL_POINTER before the request reaches the specific HAL.

Bug: 360755781
Test: atest VtsAidlHalCameraProvider_TargetTest

Change-Id: I8c558aa6b1d7c89b89506b975b4acd97b5571791