Fix json loader when conversion not implemented
Sometimes the arrow json parser fails if the `block_size` is too small and returns an `ArrowNotImplementedError: JSON conversion to struct...` error.

By increasing the block size it makes it work again.

Hopefully it should help with https://github.com/huggingface/datasets/issues/2799

I tried with the file mentioned in the issue and it worked for me
cc @lewtun can you try again from this branch ?