Adding SOAP header elements without namespace

问题

I want to add a header element in SOAP envelope. Each of my attempts throws the exception:

com.sun.xml.internal.messaging.saaj.soap.impl.HeaderImpl addHeaderElement SEVERE: SAAJ0131: HeaderElements must be namespace qualified

I need to add just a simple element without namespace inside the header. In this case, I have used a handler:

public boolean handleMessage(SOAPMessageContext context) {
    try {
        SOAPMessage message = context.getMessage();
        SOAPHeader header = message.getSOAPHeader();
        SOAPEnvelope envelope = message.getSOAPPart().getEnvelope();
        if (header == null) {
            header = envelope.addHeader();
        }
        envelope.addNamespaceDeclaration("gat", "http://schemas.datacontract.org/2004/07/Gateway.Servicios");

        QName passwordQname = header.createQName("Password", "gat");
        QName userQname = header.createQName("User", "gat");

        // Here, I’m trying to add a QName with no namespace.
        QName qNameUserCredentials = new QName(XMLConstants.NULL_NS_URI, "MyHeader");
        SOAPHeaderElement userCredentials = header.addHeaderElement(qNameUserCredentials);

        SOAPHeaderElement password = header.addHeaderElement(passwordQname);
        password.addTextNode(this.password);

        SOAPHeaderElement username = header.addHeaderElement(userQname);
        username.addTextNode(this.username);

        userCredentials.addChildElement(password);
        userCredentials.addChildElement(username);
        message.saveChanges();

        StringWriter writer = new StringWriter();
        message.writeTo(new StringOutputStream(writer));
        System.out.println("SOAP message: \n" + writer.toString());
    } catch (SOAPException e) {
        System.out.println("Error occurred while adding credentials to SOAP header." + e.getMessage());
    } catch (IOException e) {
        System.out.println("Error occurred while writing message to output stream." + e.getMessage());
    }
    return true;
}

Expected output:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:gat="http://schemas.datacontract.org/2004/07/Gateway.Servicios" xmlns:tem="http://tempuri.org/">
    <soapenv:Header>
        <MyHeader>   <-- ######## -->
           <gat:Password>da92767c161e4a62b49f23cb8e712d8e</gat:Password>
            <gat:User>3f3a1850e523fd9961cfa2f4d36bd44d</gat:User>
        </MyHeader>  <-- ######## -->
    </soapenv:Header>
    <soapenv:Body>
        <tem:ConsultarMediosPago>
            <tem:codigoPais>MX</tem:codigoPais>
        </tem:ConsultarMediosPago>
    </soapenv:Body>
</soapenv:Envelope>

The element <MyHeader></MyHeader> is what I need to add.