Stats fails with ComputeError leading to JobManagerCrashedError
While running stats job runner for teknium/OpenHermes-2.5 dataset, it fails with the following error:
```
thread '<unnamed>' panicked at crates/polars-arrow/src/array/struct_/mod.rs:117:52:
called `Result::unwrap()` on an `Err` value: ComputeError(ErrString("The children must have an equal number of values.\n                         However, the values at index 1 have a length of 2048, which is different from values at index 0, 2129."))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

```
After the exception, the worker crashes.

See full log here 
[prod-datasets-server-worker.txt](https://github.com/huggingface/datasets-server/files/14126172/prod-datasets-server-worker.txt)

Stats fails with ComputeError leading to JobManagerCrashedError: While running stats job runner for teknium/OpenHermes-2.5 dataset, it fails with the following error:
```
thread '<unnamed>' panicked at crates/polars-arrow/src/array/struct_/mod.rs:117:52:
called `Result::unwrap()` on an `Err` value: ComputeError(ErrString("The children must have an equal number of values.\n                         However, the values at index 1 have a length of 2048, which is different from values at index 0, 2129."))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

```
After the exception, the worker crashes.

See full log here 
[prod-datasets-server-worker.txt](https://github.com/huggingface/datasets-server/files/14126172/prod-datasets-server-worker.txt)
