GLTFLoader: check if material.extra is defined before assign it

examples/js/loaders/GLTFLoader.js

@@ -1669,7 +1669,7 @@ THREE.GLTFLoader = ( function () {
 				// https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#materialnormaltexture
 				_material.normalScale.x = -1;
 
-				_material.userData = material.extras;
+				if ( material.extras ) _material.userData = material.extras;
 
 				return _material;