ImageLoader: Remove url parameter check in load().

src/loaders/ImageLoader.js

@@ -17,8 +17,6 @@ ImageLoader.prototype = Object.assign( Object.create( Loader.prototype ), {
 
 	load: function ( url, onLoad, onProgress, onError ) {
 
-		if ( url === undefined ) url = '';
-
 		if ( this.path !== undefined ) url = this.path + url;
 
 		url = this.manager.resolveURL( url );