DropDownList加入空白值

  <asp:DropDownList ID="DDL_GOOD" runat="server">                     <asp:ListItem Value=""></asp:ListItem>                        <asp:ListItem Value="FG">成品   </asp:ListItem>
                        <asp:ListItem Value="WIP">半成品</asp:ListItem>
                    </asp:DropDownList>