Fix FlaxPretTrainedModel pt weights check
# What does this PR do?

[files tab](https://github.com/huggingface/transformers/pull/19133/files) should make it clear on what's being changed.

```py
if os.path.join(pretrained_model_name_or_path, WEIGHTS_NAME)
# will evaluate always to True as long as `pretrained_model_name_or_path` is a non-empty str
```
_The documentation is not available anymore as the PR was closed or merged._