jaxb xs:element ignore empty list

i have a schema file like

 <xs:complexType name="Autorisation">
    <xs:sequence>
        <xs:element name="roles" type="Roles"/>
    </xs:sequence>
 </xs:complexType>

<xs:complexType name="Roles">
    <xs:sequence>
        <xs:element name="role" type="Role" maxOccurs="unbounded" />
    </xs:sequence>
</xs:complexType>

<xs:complexType name="Role">
  <xs:sequence>
    <xs:element name="type" type="xs:int"/>
    <xs:element name="dateDebut" type="xs:dateTime"/>
    <xs:element name="dateFin" type="xs:dateTime"/>
  </xs:sequence>
</xs:complexType>

it generate something like that :

@XmlElementWrapper
@XmlElement(name = "role")
protected List<Role> roles;

When i create an xml file with JAXB.marshal(input, Output); with an empty roles list, the xml file generated contains <role/>

How can i avoid that ?

Thanks !! :)