GLTFLoader: Update comment (#25034)

examples/jsm/loaders/GLTFLoader.js

@@ -2843,7 +2843,7 @@ class GLTFParser {
 	/**
 	 * Specification: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#textures
 	 * @param {number} textureIndex
-	 * @return {Promise<THREE.Texture>}
+	 * @return {Promise<THREE.Texture|null>}
 	 */
 	loadTexture( textureIndex ) {