Change signature of ol.layer.Group#getLayers

This commit changes the signature of ol.layer.Group#getLayers from {ol.Collection.<ol.layer.Base>|undefined} to {!ol.Collection.<ol.layer.Base>}. In this way the caller is guaranteed that getLayers returns a dereferencable object.
Remove undefined from ol.Map#getLayers return type

It's already assumed that the collection is defined in ol.Map#addLayer
and ol.Map#removeLayer.