问returnUrl是否始终为空？

EN

@using MundialDeFutbol.Models
@using System.Web.Optimization
@model LoginModel

@{
    ViewBag.Title = "Login";
}

<h3 class="text-info">Login</h3>
<div class="separadorHorizontal"></div>

@using (Html.BeginForm("Login", "Usuarios", new { ReturnUrl = ViewBag.ReturnUrl }, FormMethod.Post,
    new { @class = "form-horizontal", @style = "margin-top: 20px;" }))
{
    @Html.AntiForgeryToken()

    <div id="loginErrors" class="form-group">
        <div class="col-sm-offset-2 col-sm-10 ">
            <strong>
                @Html.ValidationSummary(true, null, new { @class = "text-danger", @id = "margin-top: 10px;" })
            </strong>
        </div>
    </div>
    <div class="form-group">
        <label for="Nick" class="col-sm-2 control-label">@Html.LabelFor(model => model.Nick)</label>
        <div class="col-sm-10">
            @Html.TextBoxFor(model => model.Nick, new { @class = "form-control", @placeholder = "Nombre de usuario" })
            <strong>
                @Html.ValidationMessageFor(model => model.Nick, null, new { @class = "text-danger", @style = "display: block; margin-top: 10px;" })
            </strong>
        </div>
    </div>
    <div class="form-group">
        <label for="Contraseña" class="col-sm-2 control-label">@Html.LabelFor(model => model.Contraseña)</label>
        <div class="col-sm-10">
            @Html.TextBoxFor(model => model.Contraseña, new { @class = "form-control", @placeholder = "Escribe contraseña", @type = "password" })
            <strong>
                @Html.ValidationMessageFor(model => model.Contraseña, null, new { @class = "text-danger", @style = "display: block; margin-top: 10px;" })
            </strong>
        </div>
    </div>
    <div class="form-group">
        <div class="col-sm-offset-2 col-sm-10">
            <button type="submit" class="btn btn-default">Entrar</button>
        </div>
    </div>
}

@section scripts {
    @Scripts.Render("~/bundles/jqueryval")
}

    [AllowAnonymous]
    public ActionResult Login(string returnUrl)
    {
        ViewBag.ReturnUrl = returnUrl;
        return View();
    }

    [HttpPost]
    [AllowAnonymous] // --> Preguntar
    [ValidateAntiForgeryToken] // --> Preguntar
    public ActionResult Login(LoginModel usuario, string returnUrl)
    {
        if (ModelState.IsValid)
        {
            if (IsValid(usuario.Nick,usuario.Contraseña))
            {
                FormsAuthentication.SetAuthCookie(usuario.Nick, false);
                return Redirect(returnUrl);
            }
            else
            {
                ModelState.AddModelError("", "Usuario o contraseña incorrectos");
            }
        }
        else
        {
            var errors = ModelState.Keys.SelectMany(key => ModelState[key].Errors);
        }

        return View();
    }