xquery transformation creates empty namespace in element

I'm sorry but I guess I just don't see the mistake I'm making here. I have a camel route which returns an XML and to be able to test the output I wrote a JUnit Test which runs with SpringRunner. There I get the XML Stream from the exchange which I validate against an XSD. This works great because the XSD throws an exception because the output XML is not valid, but I don't understand why the following xquery generates an element with EMPTY NAMESPACE? See the xquery snippet (I'm sorry again I cannot provide more code):

declare default element namespace "http://www.dppgroup.com/XXXPMS";
let $cmmdoc := $doc/*:cmmdoc
, $partner := $doc/*:cmmdoc/*:information/*:partner_gruppe/*:partner
, $sequence:= fn:substring($cmmdoc/@unifier,3)
 return <ClientMMS xmlns:infra="http://www.dppgroup.com/InfraNS">
{
    for $x in $partner
    where $x[@partnerStatusCode = " "]
    return
    element {"DataGroup" } {
             <Client sequenceNumber="{$sequence}" />
    }
}

My problem is, that with this code the resulting XML contains the DataGroup-element with the following namespace definition:

<?xml version="1.0" encoding="UTF-8"?>
<ClientMMS xmlns="http://www.dppgroup.com/XXXPMS"
           xmlns:infra="http://www.dppgroup.com/InfraNS">
   <DataGroup xmlns="">
      <Client sequenceNumber="170908065609671475"/> 
   </DataGroup>
 </ClientMMS>

The snippet from the Unit-Test: I'm using jdk1.8_102

    String xml = TestDataReader.readXML("/input/info/info_in.xml", PROJECT_ENCODING);
    quelle.sendBody(xml);

    boolean valid = false;
    try {
        DocumentBuilder documentBuilder = DocumentBuilderFactory.newInstance().newDocumentBuilder();

        ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream((byte[]) archiv.getExchanges().get(1).getIn().getBody());
        Document document = documentBuilder.parse(byteArrayInputStream);

        Transformer transformer = TransformerFactory.newInstance().newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        StreamResult result = new StreamResult(new StringWriter());
        DOMSource source = new DOMSource(document);

        transformer.transform(source, result);
        String xmlString = result.getWriter().toString();
        System.out.println(xmlString);

In no XQuery introduction/tutorial/explanation I can find a reason why this happens. Can you guys please explain why the DataGroup element is not in the default namespace?

The XQuery you posted should create the result fine without the namespace undeclaration you show.

In your Java code if you want to work with XML with namespaces make sure you use a namespace aware DocumentBuilder, as the default DocumentBuilderFactory is not namespace aware make sure you set setNamespaceAware(true) on the factory before creating a DocumentBuilder with it.