`None` converted to `"None"` when loading a dataset
## Describe the bug
When loading a dataset `None` values of the type `NoneType` are converted to `'None'` of the type `str`.

## Steps to reproduce the bug
```python
from datasets import load_dataset
qasper = load_dataset("qasper", split="train", download_mode="reuse_cache_if_exists")
print(qasper[60]["full_text"]["section_name"])
```

When installing version 1.1.40, the output is
`[None, 'Introduction', 'Benchmark Datasets', ...]`

When installing from the master branch, the output is
`['None', 'Introduction', 'Benchmark Datasets', ...]`

Notice how the first element was changed from `NoneType` to `str`.

## Expected results
`None` should stay as is.

## Actual results
`None` is converted to a string.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: master
- Platform: Linux-4.4.0-19041-Microsoft-x86_64-with-glibc2.17
- Python version: 3.8.10
- PyArrow version: 4.0.1
