Object3D: Avoid serialising geometry for Sprites.

src/core/Object3D.js

@@ -678,7 +678,7 @@ Object3D.prototype = Object.assign( Object.create( EventDispatcher.prototype ),
 
 		}
 
-		if ( this.geometry !== undefined ) {
+		if ( this.isMesh || this.isLine || this.isPoints ) {
 
 			object.geometry = serialize( meta.geometries, this.geometry );