Loading of datasets changed in #3110 returns no examples 
## Describe the bug
Loading of datasets changed in https://github.com/huggingface/datasets/pull/3110 returns no examples:
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
})
```

## Steps to reproduce the bug
Load any of the datasets that were changed in https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper")

# The problem only started with the commit of #3110
load_dataset("qasper", revision="b6469baa22c174b3906c631802a7016fedea6780")
```

## Expected results
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 888
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 281
    })
})
```
Which can be received when specifying revision of the commit before https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper", revision="acfe2abda1ca79f0ce5c1896aa83b4b78af76b7d")
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.15.2.dev0 (master)
- Python version: 3.8.10
- PyArrow version: 3.0.0
