FromUriNull

public static void Register(HttpConfiguration config)
{
    config.Services.Add(typeof(System.Web.Http.ValueProviders.ValueProviderFactory),
                        new EmptyValueProviderFactory());
}

public class EmptyValueProvider : System.Web.Http.ValueProviders.IValueProvider
{
    public bool ContainsPrefix(string prefix)
    {
        return prefix != null && prefix.Length == 0;
    }

    public System.Web.Http.ValueProviders.ValueProviderResult GetValue(string key)
    {
        return null;
    }
}

public class EmptyValueProviderFactory : System.Web.Http.ValueProviders.ValueProviderFactory, 
                                         System.Web.Http.ValueProviders.IUriValueProviderFactory
{
    private static readonly EmptyValueProvider _valueProvider = new EmptyValueProvider();

    public override System.Web.Http.ValueProviders.IValueProvider GetValueProvider(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        if (actionContext.ControllerContext.RouteData.Values.Count == 0)
        {
            return _valueProvider;
        }
        return null;
    }
}