fix unbatched arrow map for iterable datasets
Fixes the bug when applying map to an arrow-formatted iterable dataset described here:

https://github.com/huggingface/datasets/issues/6833#issuecomment-2399903885

```python

from datasets import load_dataset
ds = load_dataset("rotten_tomatoes", split="train", streaming=True)
ds = ds.with_format("arrow").map(lambda x: x)
for ex in ds:
    pass
```

@lhoestq 