Fix missing info when loading some datasets from Parquet export
Fix getting the info for script-based datasets with Parquet export with a single config not named "default".

E.g.

```python
from datasets import load_dataset_builder

b = load_dataset_builder("bookcorpus")
print(b.info.features)
# should print {'text': Value(dtype='string', id=None)}
```

I fixed this by setting the default config name when there is only one config.