Ognl select 增加空选项

<form action="store.action" method="post" >        <s:select label="仓库" name="storage.storageid"  list="#request.storages" listKey="storageid" listValue="storagename" headerKey="" headerValue="" />        <s:select label="商品类别" name="productType.typeid"  list="#request.productTypes" listKey="typeid" listValue="typename" headerKey="" headerValue="" />    <s:textfield name="product.productname" label="商品名称" />    <s:select label="规格" name="product.prop2"  list="#{'350ML':'350ML','550ML':'550ML','750ML':'750ML'}" headerKey="" headerValue="" />    <s:select label="级别" name="product.prop1"  list="#{'D.O.C':'D.O.C','V.D.P':'V.D.P'}" headerKey="" headerValue="" />    <input type="submit" value="查询">    <input type="button" value="查看全部库存" onclick="javascript:window.location='store.action'" />    </form>

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headerKey="" headerValue=""