Fix crash when appending arrays to sub models in list models

The elements are still inaccessible, but at least it doesn't crash. This is
consistent with the behavior in Qt 5.0, where it didn't crash because the v8
object handle returned an empty array for the property names because the
individual array element wasn't an object.

Bug QTBUG-12117 tracks support for arrays in list models.

Task-number: QTBUG-35891
Change-Id: Ief446341344977a1473bca474ca5ba934e950468
Reviewed-by: Albert Astals Cid <albert.astals@canonical.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>