The Docs for QJsonObject in Qt 5 say:

The returned QJsonValue is QJsonValue::Undefined if the key does not exist.

However, when I do:

QJsonObject o;
QJsonValue foo = o["bar"];
qDebug() << foo;

I get:

QJsonValue(null)

And foo.isNull() == true and foo.isUndefined() == false

Tested with 5.1.1 and 5.3.