PR#3942: policy_data_from_task_args: set target to None when it doesn't exist
    
    Merges #3942
    https://pagure.io/koji/pull-request/3942
    
    Fixes: #3941
    https://pagure.io/koji/issue/3941
    build task: nonexistent buildtarget with opts.repo_id breaks policy check in make_task