emptyRegion fixes

Until marionettejs#3247 is in, the `_parent` here will skip over the `CollectionView` and trigger on any `View` above the `CollectionView`.  For that reason, currently setting the `_parent` here is a bad idea and users will have to listen to the `emptyRegion` or `emptyRegion.currentView`.  Until a CollectionView can be treated as a parent of a Region we should leave this line out 😿

Additionally the emptyRegion should be destroyed on clean up

src/next-collection-view.js

@@ -69,9 +69,7 @@ const CollectionView = Backbone.View.extend({
 
   // Create an region to show the emptyView
   _initEmptyRegion() {
-    this.emptyRegion = new Region({ el: this.el })
-
-    this.emptyRegion._parent = this;
+    this.emptyRegion = new Region({ el: this.el });
   },
 
   // Configured the initial events that the collection view binds to.
@@ -660,6 +658,7 @@ const CollectionView = Backbone.View.extend({
   // called by ViewMixin destroy
   _removeChildren() {
     this._destroyChildren();
+    this.emptyRegion.destroy();
   },
 
   // Destroy the child views that this collection view is holding on to, if any