The setValue documentation tells that empty path will "set the value of this publisher's path." However the following code will create empty subPath:

QString itemPath = "/Settings/Nokia/General/Mappings";
qDebug() << "paths:" << QValueSpaceSubscriber(itemPath).subPaths();
QValueSpacePublisher pub(itemPath);
pub.setValue("", value);
pub.sync();
qDebug() << "paths:" << QValueSpaceSubscriber(itemPath).subPaths();

prints out following:
paths: ()
paths: ("")