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/keyboardzoominteraction.js

@@ -70,7 +70,7 @@ ol.interaction.KeyboardZoom.prototype.handleMapBrowserEvent =
       var delta = (charCode == '+'.charCodeAt(0)) ? this.delta_ : -this.delta_;
       map.render();
       var view = map.getView();
-      goog.asserts.assert(goog.isDef(view));
+      goog.asserts.assert(!goog.isNull(view));
       ol.interaction.Interaction.zoomByDelta(
           map, view, delta, undefined, this.duration_);
       mapBrowserEvent.preventDefault();