fix Dataset.map when num_procs > num rows
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{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2566', 'html_url': 'https://github.com/huggingface/datasets/pull/2566', 'diff_url': 'https://github.com/huggingface/datasets/pull/2566.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2566.patch', 'merged_at': datetime.datetime(2021, 7, 1, 9, 11, 13)}
closes #2470

## Testing notes
To run updated tests:
```sh
pytest tests/test_arrow_dataset.py -k "BaseDatasetTest and test_map_multiprocessing" -s
```
With Python code (to view warning):
```python
from datasets import Dataset


dataset = Dataset.from_dict({"x": ["sample"]})
print(len(dataset))
dataset.map(lambda x: x, num_proc=10)
```
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