Make inspect.get_dataset_config_names always return a non-empty list of configs
**Is your feature request related to a problem? Please describe.**

Currently, some datasets have a configuration, while others don't. It would be simpler for the user to always have configuration names to refer to

**Describe the solution you'd like**

In that sense inspect.get_dataset_config_names should always return at least one configuration name, be it `default` or `Check___region_1` (for community datasets like `Check/region_1`).

https://github.com/huggingface/datasets/blob/c5747a5e1dde2670b7f2ca6e79e2ffd99dff85af/src/datasets/inspect.py#L161
