Fix offline mode with single config
Reported in https://github.com/huggingface/datasets/issues/4760

The cache was not able to reload a dataset with a single config form the cache if the config name is not specificed

For example

```python
from datasets import load_dataset, config

config.HF_DATASETS_OFFLINE = True

load_dataset("openai_humaneval")
```

This was due to a regression in https://github.com/huggingface/datasets/pull/6632