Validate null complex type models when query string is empty

ActionContextExtensions.cs

public static class ActionContextExtensions
{
    public static void ValidateNullModel(this HttpActionContext actionContext)
    {
        var notNullParametersDescriptors = actionContext.ActionDescriptor.GetParameters()
            .Where(p => p.GetCustomAttributes<NotNullAttribute>().Count > 0);

        foreach (var pd in notNullParametersDescriptors)
        {
            var model = actionContext.ActionArguments[pd.ParameterName];

            if (model == null)
            {
                model = Activator.CreateInstance(pd.ParameterType);
                actionContext.ActionArguments[pd.ParameterName] = model;
            }
        }
    }
}

ActionFilter.cs

public class ActionFilter : ActionFilterAttribute, IFilter
{
    public override Task OnActionExecutingAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
    {
        actionContext.ValidateNullModel();
    }
}

ExampleController.cs

[HttpGet]
[Route("")]
public IHttpActionResult Get([FromUri][NotNull] ComplexType request)
{
    return Ok();
}

NotNullAttribute.cs

[AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
public class NotNullAttribute : Attribute
{
    
}