Move the nullFunction to ol/functions

And rename it to `UNDEFINED`.

src/ol/index.js

@@ -93,14 +93,6 @@ export function inherits(childCtor, parentCtor) {
 }
 
 
-/**
- * A reusable function, used e.g. as a default for callbacks.
- *
- * @return {undefined} Nothing.
- */
-export function nullFunction() {}
-
-
 /**
  * Counter for getUid.
  * @type {number}