I have problem. When i didn't search something in dropdown It has got default selected value=0. And its saving my database. If it can send value"NULL" not will save in my database. How can i send default value "NULL"?

  <div class="form-group">
        <label class="control-label col-md-2"></label>
        <div class="col-md-8">
            @*@Html.DropDownList("CustomerID", null, new { @class = "form-control", required = "required", placeholder = "Müşteri Seçiniz" })*@
            @Html.DropDownList("CustomerID", Enumerable.Empty<SelectListItem>(), "", new { @multiple = "multiple", @id = "CustomerID", @class = "form-control" })
            @Html.ValidationMessageFor(model => model.CustomerID, "", new { @class = "text-danger", style="color:red"})
            <p style="color:red;margin-bottom:-3px" id="errorpersonel"></p>
        </div>
    </div>
<script language="javascript" type="text/javascript">

$(document).ready(function () {
    debugger;
    $("#CustomerID").pqSelect({
        singlePlaceholder: '', width: '92%'
    });

    $.ajax({

        url: "/TicketDashboard/GetCustomer",
        method: "Post",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (data) {


            $("#CustomerID").empty();

            $.each(data, function (index, value) {
                debugger;

                $('#CustomerID').append('<option value="' + value.CustomerID + '">' + value.CustomerName + '</option>');

            });

            $("#CustomerID").pqSelect("refreshData");

            $('span[data-valmsg-for="OpportunityAttachmentViewModel.CCMail"]').html('Boş geçilemez.');
        }
    });
});

$("#CustomerID").pqSelect({
    singlePlaceholder: '',
    multiplePlaceholder: 'Müşteri Seçiniz',
    maxSelect: 1,
}).on("change", function (evt) {



    pccmail = $(this).val();

    if (pccmail != "") {
        $('span[data-valmsg-for="OpportunityAttachmentViewModel.CCMail"]').html('');



    } else {
        $('span[data-valmsg-for="OpportunityAttachmentViewModel.CCMail"]').html('Boş geçilemez');
    }
    $('[id^=CCMailinput]').val(pccmail);
});

Its my codes. How can i change selected value "0"? It should be "NULL" when i sent form