Как в xml тег с датой типа XMLGregorianCalendar оставить пустым?

public class CoordinateStatusData {

    @XmlElement(name = "ResponseDate", required = true, nillable = true)
    @XmlSchemaType(name = "dateTime")
    protected XMLGregorianCalendar responseDate;
    @XmlElement(name = "PlanDate", required = true, nillable = true)
    @XmlSchemaType(name = "dateTime")
    protected XMLGregorianCalendar planDate;
    @XmlElement(name = "Status", required = true)
    protected StatusType status;
    @XmlElement(name = "Responsible", required = true)
    protected Person responsible;
    @XmlElement(name = "Documents")
    protected ArrayOfServiceDocument documents;
    @XmlElement(name = "Contacts")
    protected ArrayOfBaseDeclarant contacts;
    @XmlElement(name = "Note")
    protected String note;
    @XmlElement(name = "Result")
    protected RequestResult result;
    @XmlElement(name = "ServiceNumber", required = true)
    protected String serviceNumber;
    @XmlElement(name = "Reason")
    protected DictionaryItem reason;
    @XmlElement(name = "Department", required = true)
    protected Department department;
    @XmlElement(name = "CreatedByDepartment")
    protected Department createdByDepartment;
    @XmlElement(name = "StatusId")
    protected String statusId;