Make ol.Map#setView do not accept undefined

With this change map.setView(null) is permitted, but map.setView(undefined) is not. map.getView now always returns a defined value, which may be null is setView was called with null.

src/ol/interaction/dragzoominteraction.js

@@ -54,7 +54,7 @@ goog.inherits(ol.interaction.DragZoom, ol.interaction.DragBox);
 ol.interaction.DragZoom.prototype.onBoxEnd = function() {
   var map = this.getMap();
   var view = map.getView();
-  goog.asserts.assert(goog.isDef(view));
+  goog.asserts.assert(!goog.isNull(view));
   var extent = this.getGeometry().getExtent();
   var center = ol.extent.getCenter(extent);
   var size = map.getSize();