如何让下拉菜单 选择默认为空？？？

         如下图：作业负责人添加时默认为登录的人员，不能为空，如何修改代码改为默认为空的？？

                       
<%
          if request.Cookies("shiwei_id")=1 then
            sql="select * from login order by id_zu,id"
            set rs_login=conn.execute(sql)
          else
            sql="select * from login where id="&request.Cookies("shiwei_id")
            set rs_login=conn.execute(sql)      
          end if
          if rs_login.eof then
          %>
          <script language="javascript">
            alert("请先添加员工！")
            window.location.href="../system/user_add.asp"
          </script>
          <%
          response.end
          else
          %>
          <select name="id_login">
          <%
          do while rs_login.eof=false
          %>
            <option value="<%=rs_login("id")%>"<%if trim(cstr(rs_login("id")))=trim(cstr(request.Cookies("shiwei_id"))) then%> selected="selected"<%end if%>><%=rs_login("username")%> </option>
          <%
            rs_login.movenext
          loop
          %>
          </select>
          <%
          end if
          %>        
这是这段代码