`None` replaced by `[]` after first batch in map
It looks like this is because of this behavior in pyarrow:
```python
import pyarrow as pa

arr = pa.array([None, [0]])
reconstructed_arr = pa.ListArray.from_arrays(arr.offsets, arr.values)
print(reconstructed_arr.to_pylist())
# [[], [0]]
```

It seems that `arr.offsets` can reconstruct the array properly, but an offsets array with null values can:
```python
fixed_offsets = pa.array([None, 0, 1])
fixed_arr = pa.ListArray.from_arrays(fixed_offsets, arr.values)
print(fixed_arr.to_pylist())
# [None, [0]]

print(arr.offsets.to_pylist())
# [0, 0, 1]
print(fixed_offsets.to_pylist())
# [None, 0, 1]
```
EDIT: this is because `arr.offsets` is not enough to reconstruct the array, we also need the validity bitmap
Sometimes `None` can be replaced by `[]` when running map:

```python
from datasets import Dataset

ds = Dataset.from_dict({"a": range(4)})
ds = ds.map(lambda x: {"b": [[None, [0]]]}, batched=True, batch_size=1, remove_columns=["a"])
print(ds.to_pandas())
#              b
# 0  [None, [0]]
# 1    [[], [0]]
# 2    [[], [0]]
# 3    [[], [0]]
```

This issue has been experienced when running the `run_qa.py` example from `transformers` (see issue https://github.com/huggingface/transformers/issues/15401)

This can be due to a bug in when casting `None` in nested lists. Casting only happens after the first batch, since the first batch is used to infer the feature types.

cc @sgugger 
`None` replaced by `[]` after first batch in map 
 Sometimes `None` can be replaced by `[]` when running map:

```python
from datasets import Dataset

ds = Dataset.from_dict({"a": range(4)})
ds = ds.map(lambda x: {"b": [[None, [0]]]}, batched=True, batch_size=1, remove_columns=["a"])
print(ds.to_pandas())
#              b
# 0  [None, [0]]
# 1    [[], [0]]
# 2    [[], [0]]
# 3    [[], [0]]
```

This issue has been experienced when running the `run_qa.py` example from `transformers` (see issue https://github.com/huggingface/transformers/issues/15401)

This can be due to a bug in when casting `None` in nested lists. Casting only happens after the first batch, since the first batch is used to infer the feature types.

cc @sgugger  
 It looks like this is because of this behavior in pyarrow:
```python
import pyarrow as pa

arr = pa.array([None, [0]])
reconstructed_arr = pa.ListArray.from_arrays(arr.offsets, arr.values)
print(reconstructed_arr.to_pylist())
# [[], [0]]
```

It seems that `arr.offsets` can reconstruct the array properly, but an offsets array with null values can:
```python
fixed_offsets = pa.array([None, 0, 1])
fixed_arr = pa.ListArray.from_arrays(fixed_offsets, arr.values)
print(fixed_arr.to_pylist())
# [None, [0]]

print(arr.offsets.to_pylist())
# [0, 0, 1]
print(fixed_offsets.to_pylist())
# [None, 0, 1]
```
EDIT: this is because `arr.offsets` is not enough to reconstruct the array, we also need the validity bitmap