Fix check_all_models_are_tested
# What does this PR do?

The block from L396 to L398 should be in the `else` block if I understand correctly. 

https://github.com/huggingface/transformers/blob/8d3f952adb8c98cec2ea1f59bb7acfbc08232381/utils/check_repo.py#L394-L398

Otherwise, when a model has no test file, I get errors like below, and the program stops immediately.
(with `test_file = []` passed to `check_models_are_tested`)

```python
  File "/home/yih_dar_huggingface_co/transformers/utils/check_repo.py", line 362, in check_models_are_tested
    tested_models = find_tested_models(test_file)
  File "/home/yih_dar_huggingface_co/transformers/utils/check_repo.py", line 343, in find_tested_models
    with open(os.path.join(PATH_TO_TESTS, test_file), "r", encoding="utf-8", newline="\n") as f:
  File "/usr/lib/python3.9/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File "/usr/lib/python3.9/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'list'
```
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