No callback from QQuick3DLightmapBaker::bake() if there are no models set up for baking

QQuick3DLightmapBaker::Callback is not called after baking is started with QQuick3DLightmapBaker::bake(callback), if there are no models included in baking (i.e. no models with usedInBakedLighting == true). It should send at the very least "Complete" or "Cancelled" callback instead of just not doing anything. An informative "Progress" callback about the situation wouldn't hurt, either.

For now QDS just assumes there were no bakeable content if no callbacks are received.