`GeneratorBasedBuilder` does not support `None` values
Hi,

thanks for reporting and providing a minimal reproducible example. 

This line of the PR I've linked in our discussion on the Forum will add support for `None` values:
https://github.com/huggingface/datasets/blob/a53de01842aac65c66a49b2439e18fa93ff73ceb/src/datasets/features/features.py#L835

I expect that PR to be merged soon.
## Describe the bug
`GeneratorBasedBuilder` does not support `None` values.

## Steps to reproduce the bug
See [this repository](https://github.com/pavel-lexyr/huggingface-datasets-bug-reproduction) for minimal reproduction.

## Expected results
Dataset is initialized with a `None` value in the `value` column.

## Actual results
```
Traceback (most recent call last):
  File "main.py", line 3, in <module>
    datasets.load_dataset("./bad-data")
  File ".../datasets/load.py", line 1632, in load_dataset
    builder_instance.download_and_prepare(
  File ".../datasets/builder.py", line 607, in download_and_prepare
    self._download_and_prepare(
  File ".../datasets/builder.py", line 697, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File ".../datasets/builder.py", line 1103, in _prepare_split
    example = self.info.features.encode_example(record)
  File ".../datasets/features/features.py", line 1033, in encode_example
    return encode_nested_example(self, example)
  File ".../datasets/features/features.py", line 808, in encode_nested_example
    return {
  File ".../datasets/features/features.py", line 809, in <dictcomp>
    k: encode_nested_example(sub_schema, sub_obj) for k, (sub_schema, sub_obj) in utils.zip_dict(schema, obj)
  File ".../datasets/features/features.py", line 855, in encode_nested_example
    return schema.encode_example(obj)
  File ".../datasets/features/features.py", line 299, in encode_example
    return float(value)
TypeError: float() argument must be a string or a number, not 'NoneType'
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.15.1
- Platform: Linux-5.4.0-81-generic-x86_64-with-glibc2.29
- Python version: 3.8.10
- PyArrow version: 6.0.0