JSON lines with missing struct fields raise TypeError: Couldn't cast array
Hello,

I have still the same issue when loading the dataset with the new version:
[https://huggingface.co/datasets/wikimedia/structured-wikipedia/discussions/5](https://huggingface.co/datasets/wikimedia/structured-wikipedia/discussions/5)

I have downloaded and unzipped the wikimedia/structured-wikipedia dataset locally but when loading I have the same issue.

```
import datasets

dataset = datasets.load_dataset("/gpfsdsdir/dataset/HuggingFace/wikimedia/structured-wikipedia/20240916.fr")
```
```
TypeError: Couldn't cast array of type
struct<content_url: string, width: int64, height: int64, alternative_text: string>
to
{'content_url': Value(dtype='string', id=None), 'width': Value(dtype='int64', id=None), 'height': Value(dtype='int64', id=None)}

The above exception was the direct cause of the following exception:
```
My version of datasets is 3.0.1
JSON lines with missing struct fields raise TypeError: Couldn't cast array of type.

See example: https://huggingface.co/datasets/wikimedia/structured-wikipedia/discussions/5

One would expect that the struct missing fields are added with null values.