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/mousewheelzoominteraction.js

@@ -101,7 +101,7 @@ ol.interaction.MouseWheelZoom.prototype.doZoom_ = function(map) {
   var delta = goog.math.clamp(this.delta_, -maxDelta, maxDelta);
 
   var view = map.getView();
-  goog.asserts.assert(goog.isDef(view));
+  goog.asserts.assert(!goog.isNull(view));
 
   map.render();
   ol.interaction.Interaction.zoomByDelta(map, view, -delta, this.lastAnchor_,