Replace goog.nullFunction with ol.nullFunction

src/ol/renderer/layerrenderer.js

@@ -4,6 +4,7 @@ goog.require('goog.asserts');
 goog.require('goog.events');
 goog.require('goog.events.EventType');
 goog.require('goog.functions');
+goog.require('ol');
 goog.require('ol.ImageState');
 goog.require('ol.Observable');
 goog.require('ol.TileRange');
@@ -47,7 +48,7 @@ goog.inherits(ol.renderer.Layer, ol.Observable);
  * @return {T|undefined} Callback result.
  * @template S,T
  */
-ol.renderer.Layer.prototype.forEachFeatureAtCoordinate = goog.nullFunction;
+ol.renderer.Layer.prototype.forEachFeatureAtCoordinate = ol.nullFunction;
 
 
 /**