`path` is `None` when downloading a custom audio dataset from the Hub
### Describe the bug

I've created an [audio dataset](https://huggingface.co/datasets/lewtun/audio-test-push) using the `audiofolder` feature desribed in the [docs](https://huggingface.co/docs/datasets/audio_dataset#audiofolder) and then pushed it to the Hub. 

Locally, I can see the `audio.path` feature is of the expected form `path/to/data_dir`, but when I download the dataset from the Hub, I see `audio.path` is `None`

Here's an example:

```python
from datasets import load_dataset

ds = load_dataset("lewtun/audio-test-push")
ds["train"][0]
# {
#     "audio": {
#         "path": None,    <-- Is this expected?
#         "array": array(
#             [
#                 3.97140226e-07,
#                 7.30310290e-07,
#                 7.56406735e-07,
#                 ...,
#                 -1.19636677e-01,
#                 -1.16811886e-01,
#                 -1.12441722e-01,
#             ]
#         ),
#         "sampling_rate": 44100,
#     },
#     "song_id": 0,
#     "genre_id": 0,
#     "genre": "Electronic",
# }
```

Is this expected behaviour? If yes, feel free to close this issue as it's not a true bug then :)

### Steps to reproduce the bug

1. Create an audio dataset with the `audiofolder` feature
2. Push the dataset to the Hub with `push_to_hub()`
3. Download the Hub dataset and inspect the `audio.path` feature

### Expected behavior

`audio.path` points to the file associated with the audio data

### Environment info

- `datasets` version: 2.6.2.dev0
- Platform: macOS-10.16-x86_64-i386-64bit
- Python version: 3.8.13
- PyArrow version: 9.0.0
- Pandas version: 1.5.1