Modelstate stays valid even if the required attributes are empty in the .NET framework web API 4.7.2

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C#

.NET

I have a strange situation, and none of the existing solutions help

I have this model in the .NET Framework Web API

public partial class RequestData
{
    public int Id { get; set; }
    [Required(AllowEmptyStrings = false)]
    public string AccountName { get; set; }
    [Required(AllowEmptyStrings = false)]
    public string VendorName { get; set; }
    [Required]
    public string Memo { get; set; }
    public Nullable<bool> IsToBePrinted { get; set; }
    [Required]
    public string AccountType { get; set; }
    [Required]
    public Nullable<double> Amount { get; set; }
    public Nullable<bool> IsProcessed { get; set; }
    public string Ticket { get; set; }
    public string Response { get; set; }
    public Nullable<System.DateTime> Updated { get; set; }
    [Required]
    public Nullable<int> RequestType { get; set; }
    [Required]
    public string TxnId { get; set; }
    [Required]
    public string EditSequence { get; set; }
    [Required]
    public string ListID { get; set; }
    [Required]
    public string TxnLineId { get; set; }
}

I have a POST controller that uses this model

[HttpPost]
public ActionResult PostNewRequest([FromBody] RequestData requestData)
{
    try
    {

        if (!ModelState.IsValid) {
            //Your code if the field is invalid
        } else {
            //Your code if the field is valid
        }

But when I send empty values for the required fields (I use SOAP UI) and those values come to a controller as NULL values, the ModelState stays valid

What am I doing wrong, and how else can I take advantage of the Required attributes/annotators?

Thank you very much in advance

What I have tried:

I tried to use the filtering functionality using the below code buy no success

public class ValidateModelAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        if (!actionContext.ModelState.IsValid) {
            actionContext.Response = actionContext.Request.CreateErrorResponse(
                HttpStatusCode.BadRequest, actionContext.ModelState);
        }
    }
}