How do I check each list item in viewBag is null or not null while looping

I am trying to show the viewbag items in a carousal. Inside foreach how do I check each list item in viewBag is null or not null before assigning the value to imgSrc.

   @{
                                var first = true;
                                foreach (var item in ViewBag.carousel)
              `enter code here`                  {
                                    string filepath = System.Configuration.ConfigurationManager.AppSettings["Carousal"].ToString();
                                    var imgSrc = "";
                                    if (item!= "0")
                                    {
                                        imgSrc = filepath + item;
                                    }
                                    else
                                    {
                                        imgSrc = filepath + "DefaultProduct.Jpg";
                                    }
                                    <div class="carousel-item @(first?Html.Raw("active"):Html.Raw(""))">
                                        <img class="d-block w-100" src="imgSrc" alt="">
                                    </div>
                                    first = false;
                                }