Return empty (null) JSON result if StatusCode 400 in ASP.Net MVC

This is Web API result return with Json data and return without Json data

This is the Models 

using System.Collections.Generic;
using System.Text;
    
namespace App1Client.Model.Login
{
    public class User
    {
        public string userId { get; set; }
    }
    
    public class LoginApiResponseModel
    {
        public string authenticationToken { get; set; }
        public User user { get; set; }
    }
}

This is my code so far 

If StatusCode=400, I don't know how to return Json with null

public async Task<LoginApiResponseModel> AuthenticateUserAsync(string phonenumber, string password)
{
    try
    {
        LoginApiRequestModel loginRequestModel = new LoginApiRequestModel()
        {
            Email = phonenumber,
            Password = password
 
        };
        var content = new StringContent(JsonConvert.SerializeObject(loginRequestModel), Encoding.UTF8, "application/json");               
        var response = await client.PostAsync("api/token", content);
            
        if (response.IsSuccessStatusCode)
        {
            using (var stream = await response.Content.ReadAsStreamAsync())
            using (var reader = new StreamReader(stream))
            using (var json = new JsonTextReader(reader))
            {
                var jsoncontent = _serializer.Deserialize<LoginApiResponseModel>(json);
                Preferences.Set("authToken", jsoncontent.authenticationToken);
                return jsoncontent;
            }
        }
        else
        {
            using (var stream = await response.Content.ReadAsStreamAsync())
            using (var reader = new StreamReader(stream))
            using (var json = new JsonTextReader(reader))
            {
                var jsoncontent = _serializer.Deserialize<LoginApiResponseModel>(json);
                Preferences.Set("authToken", jsoncontent.authenticationToken);
                return jsoncontent;
            }
        }
    }
    catch (Exception ex)
    {
        return null;
    }
}

Please help. I'm stuck