fix tokenizer isfile checking code (#2424)

* fix tokenizer isfile checking code

* fix code  style

Author: wj-Mcat

paddlenlp/transformers/tokenizer_utils_base.py

@@ -1619,7 +1619,7 @@ def convert_added_tokens(obj):
             # does include a vocab file path in it. However, if the vocab file
             # path included in json does not exist, such as was deleted, to make
             # it still work, use the vocab file under this dir.
-            elif not os.path.isfile(init_kwargs[args_name],
+            elif not os.path.isfile(init_kwargs[args_name] or
                                     '') and os.path.isfile(file_path):
                 init_kwargs[args_name] = file_path
         # TODO(guosheng): avoid reduplication of position args and key word args