Title: `Dataset` unexpected changed dict data and may cause error
Author: scruel (NONE)
State: open
Created: 2024-02-28T05:30:10Z

### Describe the bug

Will unexpected get keys with `None` value in the parsed json dict.

### Steps to reproduce the bug

```jsonl test.jsonl
{"id": 0, "indexs": {"-1": [0, 10]}}
{"id": 1, "indexs": {"-1": [0, 10]}}
```

```python
dataset = Dataset.from_json('.test.jsonl')
print(dataset[0])
```

Result:
```
{'id': 0, 'indexs': {'-1': [...], '-2': None, '-3': None, '-4': None, '-5': None, '-6': None, '-7': None, '-8': None, '-9': None, ...}}
```

Those keys with `None` value will unexpected appear in the dict.


### Expected behavior

Result should be
```
{'id': 0, 'indexs': {'-1': [0, 10]}}
```

### Environment info

- `datasets` version: 2.16.1
- Platform: Linux-6.5.0-14-generic-x86_64-with-glibc2.35
- Python version: 3.11.6
- `huggingface_hub` version: 0.20.2
- PyArrow version: 14.0.2
- Pandas version: 2.1.4
- `fsspec` version: 2023.10.0

Comments:
If `test.jsonl` contains more lines like:
```
{"id": 0, "indexs": {"-1": [0, 10]}}
{"id": 1, "indexs": {"-1": [0, 10]}}
{"id": 2, "indexs": {"-2": [0, 10]}}
...
{"id": n, "indexs": {"-9999": [0, 10]}}
```

`Dataset.from_json` will just raise an error:
```
An error occurred while generating the dataset
TypeError: Couldn't cast array of type
struct<-5942: list<item: int64>, -5943: list<item: int64>, -5944: list<item: int64>, -5945: list<item: int64>, -5946: list<item: int64>, -5947: list<item: int64>, -5948: list<item: int64>, -5949: list<item: int64>: ...
to
{... '-5312': Sequence(feature=Value(dtype='int64', id=None), length=-1, id=None), '-5313': Sequence(feature=Value(dtype='int64', id=None), length=-1, id=None)}

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/runpy.py", line 198, in _run_module_as_main
    return _run_code(code, main_globals, None,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/runpy.py", line 88, in _run_code
    exec(code, run_globals)
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/adapter/../../debugpy/launcher/../../debugpy/__main__.py", line 39, in <module>
    cli.main()
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/adapter/../../debugpy/launcher/../../debugpy/../debugpy/server/cli.py", line 430, in main
    run()
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/adapter/../../debugpy/launcher/../../debugpy/../debugpy/server/cli.py", line 284, in run_file
    runpy.run_path(target, run_name="__main__")
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_runpy.py", line 321, in run_path
    return _run_module_code(code, init_globals, run_name,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_runpy.py", line 135, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/home/scruel/.vscode-server/extensions/ms-python.debugpy-2024.0.0-linux-x64/bundled/libs/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_runpy.py", line 124, in _run_code
    exec(code, run_globals)
  File "/home/scruel/Code/Python/Working/llm-memory/data_reader.py", line 120, in <module>
    reader = SnippetReader(jsonl_path, npy_path)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/scruel/Code/Python/Working/llm-memory/data_reader.py", line 85, in __init__
    self._dataset = Dataset.from_json(jsonl_path, features=)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/arrow_dataset.py", line 1130, in from_json
    ).read()
      ^^^^^^
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/io/json.py", line 59, in read
    self.builder.download_and_prepare(
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/builder.py", line 1005, in download_and_prepare
    self._download_and_prepare(
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/builder.py", line 1100, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/builder.py", line 1860, in _prepare_split
    for job_id, done, content in self._prepare_split_single(
  File "/home/scruel/mambaforge/envs/vae/lib/python3.11/site-packages/datasets/builder.py", line 2016, in _prepare_split_single
    raise DatasetGenerationError("An error occurred while generating the dataset") from e
datasets.exceptions.DatasetGenerationError: An error occurred while generating the dataset
```

Hi! Our JSON parser expects all examples/rows to share the same set of columns (applies to nested columns, too), hence the error. 

To read the `index` column, we would have to manually cast the input to PyArrow's `pa.map_` type, but this requires a more thorough investigation, as `pa.map_` has limited support in PyArrow.