您的代码有几个问题：

1. 指令需要返回值，以便可以绑定到相应的表单控件：

export class NullValueDirectiveDirective {

   @Output('EmptyToNull') response = new EventEmitter<string>();

   @HostListener('keyup', ['$event']) onKeyDowns(event: KeyboardEvent) {
      this.response.emit(null);
   }
}

<input  formControlName="AuthorityNum" 
(EmptyToNull) = "form.controls.AuthorityNum.value = $event">