Move the nullFunction to ol/functions

And rename it to `UNDEFINED`.

src/ol/source/Source.js

@@ -1,7 +1,8 @@
 /**
  * @module ol/source/Source
  */
-import {inherits, nullFunction} from '../index.js';
+import {inherits} from '../index.js';
+import {UNDEFINED} from '../functions.js';
 import BaseObject from '../Object.js';
 import {get as getProjection} from '../proj.js';
 import SourceState from '../source/State.js';
@@ -89,7 +90,7 @@ Source.prototype.adaptAttributions_ = function(attributionLike) {
  * @return {T|undefined} Callback result.
  * @template T
  */
-Source.prototype.forEachFeatureAtCoordinate = nullFunction;
+Source.prototype.forEachFeatureAtCoordinate = UNDEFINED;
 
 
 /**

src/ol/source/Tile.js

@@ -1,7 +1,8 @@
 /**
  * @module ol/source/Tile
  */
-import {inherits, nullFunction} from '../index.js';
+import {inherits} from '../index.js';
+import {UNDEFINED} from '../functions.js';
 import TileCache from '../TileCache.js';
 import TileState from '../TileState.js';
 import Event from '../events/Event.js';
@@ -303,7 +304,7 @@ TileSource.prototype.refresh = function() {
  * @param {number} y Tile coordinate y.
  * @param {ol.proj.Projection} projection Projection.
  */
-TileSource.prototype.useTile = nullFunction;
+TileSource.prototype.useTile = UNDEFINED;
 
 
 /**

src/ol/source/Vector.js

@@ -2,7 +2,7 @@
  * @module ol/source/Vector
  */
 
-import {getUid, inherits, nullFunction} from '../index.js';
+import {getUid, inherits} from '../index.js';
 import Collection from '../Collection.js';
 import CollectionEventType from '../CollectionEventType.js';
 import ObjectEventType from '../ObjectEventType.js';
@@ -13,7 +13,7 @@ import Event from '../events/Event.js';
 import EventType from '../events/EventType.js';
 import {containsExtent, equals} from '../extent.js';
 import {xhr} from '../featureloader.js';
-import {TRUE} from '../functions.js';
+import {TRUE, UNDEFINED} from '../functions.js';
 import {all as allStrategy} from '../loadingstrategy.js';
 import {isEmpty, getValues} from '../obj.js';
 import Source from '../source/Source.js';
@@ -75,7 +75,7 @@ const VectorSource = function(opt_options) {
    * @private
    * @type {ol.FeatureLoader}
    */
-  this.loader_ = nullFunction;
+  this.loader_ = UNDEFINED;
 
   /**
    * @private