Return Status::ILLEGAL_ARGUMENT if callback is nullptr

AIDL documentation states that ICameraProvider::setCallback should
return Status::ILLEGAL_ARGUMENT if the callback passed is a null
pointer. The current implementation incorrectly returns OK even if the
callback is null. This CL adds a null checks and returns
Status::ILLEGAL_ARGUMENT if required.

Bug: 233221359
Test: atest VtsAidlHalCameraProvider_TargetTest
Change-Id: I2d705262df0f7acd403cd1eeae1bdc09a18565d2