Newly created QJsonObject returns Null instead of Undefined in value()

The documented behavior for QJsonObject::value(const QString &key) is that an Undefined QJsonValue is returned when the key does not exist.
When a QJsonObject is uninitialized (which means the d member is still null), the returned value is Null instead of Undefined.
The behavior is caused directly by the QJsonObject::value() method, containing these lines:

  QJsonValue QJsonObject::value(const QString &key) const
  {
      if (!d)
          return QJsonValue();
      [...]
  }