Remove goog.isDefAndNotNull test on ol.layer.Group#getLayers result

The return type of the function is {!ol.Collection.<ol.layer.Base>}

src/ol/layer/layergroup.js

@@ -96,7 +96,6 @@ ol.layer.Group.prototype.handleLayersChanged_ = function(event) {
   }
 
   var layers = this.getLayers();
-  if (goog.isDefAndNotNull(layers)) {
   this.listenerKeys_ = {
     'add': goog.events.listen(layers, ol.CollectionEventType.ADD,
         this.handleLayersAdd_, false, this),
@@ -113,7 +112,6 @@ ol.layer.Group.prototype.handleLayersChanged_ = function(event) {
             [ol.ObjectEventType.PROPERTYCHANGE, goog.events.EventType.CHANGE],
             this.handleLayerChange_, false, this);
   }
-  }
 
   this.changed();
 };