title: Exclude "The value 'null' is not valid for ..." in ActionFilterAttribute
body: <p>In a webapi project we have a model like:</p>
  
  <pre><code>public class Person
  {
      public string Name { get; set; }
  
      public Guid? Id { get; set; }
  }
  </code></pre>
  
  <p>We have configured validation of parameters and we do some checks using ActionFilterAttribute:</p>
  
  <pre><code>public class ModelActionFilter : ActionFilterAttribute
  {
      public override void OnActionExecuting(HttpActionContext actionContext)
      {
          (...)
          var modelState = actionContext.ModelState;
          if (modelState.IsValid == false)
          {
              actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, modelState);
          }
  
          base.OnActionExecuting(actionContext);
      }
  }
  </code></pre>
  
  <p>The problem is that, doing a call like: <a href="https://localhost/person?Id=null&amp;name=" rel="nofollow noreferrer">https://localhost/person?Id=null&amp;name=</a>'John', creates an error like:</p>
  
  <pre><code>The value 'null' is not valid for Id.
  </code></pre>
  
  <p>We have made the Id field nullable in the first place because we want to allow calls like the one above. Still, the validator complains. Is there any clean way to exclude this error? </p>
  
  <p>I could go and iterate through the list of errors and exclude this one, but it feels really wrong.</p>