Fix unreliable behavior of array methods on qml list properties

Fix unreliable behavior of array methods on qml list properties

Array methods such as forEach rely on the hasProperty boolean of getIndexed to
be set appropriately. Some getIndexed implementation - such as the
QQmlListProperty one - didn't initialize it correctly and therefore the
behavior was undefined.

Task-number: QTBUG-38088
Change-Id: I34bc3136d8cc2bc280397d0c4d5051e7d72269e8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>