Fix json loader when conversion not implemented
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["And we're already at PR number 3,000 ! ^^"
 'Thank you so much for fixing this @lhoestq 😍 ! I just tested the branch out and it works like a charm!']
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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 ?
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