I met a problem when JAXB unmarshalling xml data.

JAXB throws exception when unmarshalling empty value for `int`, `double` or `date` attribute from xml. For example, it throws `java.lang.NumberFormatException` when it unmarshals the following xml data.

```
<sku displayName="iphone" price=""/>

```

The following is my schema:

```
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="sku" type="SkuType" maxOccurs="unbounded"/>
    <xs:complexType name="SkuType">
        <xs:attribute name="displayName" type="xs:string" use="required"/>
        <xs:attribute name="price" type="xs:double" use="required"/>
        <xs:attribute name="startDate" type="xs:dateTime" use="optional"/>
        <xs:attribute name="minimumOrderQty" type="xs:integer" use="optional"/>
    </xs:complexType>
</xs:schema>

```

Sorry for the messy xml. I can't type "left angle" sign in input. Can anyone help me out?

Thanks a lot.