Allow empty namespace URI

W3C Namespaces Standard states that "The attribute value in a default
namespace declaration MAY be empty" (secion 6.2). Analysis and fix
thanks to Eugenio Rustico.

Task-number: QTBUG-63538
Change-Id: Icd8d4df639b9737d8e0d215bf2bea56fe1e161ac
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/xml/qxmlstream.cpp

@@ -3708,7 +3708,6 @@ void QXmlStreamWriter::writeEntityReference(const QString &name)
 void QXmlStreamWriter::writeNamespace(const QString &namespaceUri, const QString &prefix)
 {
     Q_D(QXmlStreamWriter);
-    Q_ASSERT(!namespaceUri.isEmpty());
     Q_ASSERT(prefix != QLatin1String("xmlns"));
     if (prefix.isEmpty()) {
         d->findNamespace(namespaceUri, d->inStartElement);