QQmlMetaTypeInterface should have a non-null metaObject

QQmlMetaTypeInterface currently has its metaObject always set to nullptr. However, if we create a QQmlVMEMetaObject anyway in QQmlObjectCreator::populateInstance, we might want to consider setting metaObject to it.

This avoids for instance some weirdness in convert, where dynamic metatypes currently lead to us encountering metatypes with QMetaType::PointerToQObject set which don't have a metaObject.

Another use case is being able to use inherits with types originating from QML.