Update train_util.py, bug fix

Author: nandometzger

library/train_util.py

@@ -1489,7 +1489,7 @@ def read_caption(img_path, caption_extension, enable_wildcard):
         def load_dreambooth_dir(subset: DreamBoothSubset):
             if not os.path.isdir(subset.image_dir):
                 logger.warning(f"not directory: {subset.image_dir}")
-                return [], []
+                return [], [], []
 
             info_cache_file = os.path.join(subset.image_dir, self.IMAGE_INFO_CACHE_FILE)
             use_cached_info_for_subset = subset.cache_info