Fix: Missing a MetadataConfigs init when the repo has a `datasets_info.json` but no README
When I try to push to an arrow repo (can provide the link on Slack), it uploads the files but fails to update the metadata, with
```
  File "app.py", line 123, in add_new_eval
    eval_results[level].push_to_hub(my_repo, token=TOKEN, split=SPLIT)
  File "blabla_my_env_path/lib/python3.10/site-packages/datasets/arrow_dataset.py", line 5501, in push_to_hub
    if not metadata_configs:
UnboundLocalError: local variable 'metadata_configs' referenced before assignment
```

This fixes it.