在MVC 3中,如何使默认模型绑定器将空字符串视为"空"而不是"空"？

public class EmptyStringModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        object result = base.BindModel(controllerContext, bindingContext);

        if (result is string resultString)
        {
            if (string.IsNullOrEmpty(resultString))
            {
                return "空";
            }
        }

        return result;
    }
}

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RegisterRoutes(RouteTable.Routes);

    ModelBinders.Binders.Add(typeof(string), new EmptyStringModelBinder());
}