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/doubleclickzoominteraction.js

@@ -50,7 +50,7 @@ ol.interaction.DoubleClickZoom.prototype.handleMapBrowserEvent =
     var anchor = mapBrowserEvent.coordinate;
     var delta = browserEvent.shiftKey ? -this.delta_ : this.delta_;
     var view = map.getView();
-    goog.asserts.assert(goog.isDef(view));
+    goog.asserts.assert(!goog.isNull(view));
     ol.interaction.Interaction.zoomByDelta(
         map, view, delta, anchor, this.duration_);
     mapBrowserEvent.preventDefault();