fix: fix seg fault in concat_str of empty series

[reswqa]

This fixes #11701.

[orlp]

I don't think this fully addresses the problem. Why did a segfault occur? My implementation was entirely in safe code. Something else needs to be changed.

[reswqa]

@orlp The segment fault caused by unsafe { arr.get_unchecked(arr_idx) } where arr has length 0, and arr_idx is 0, seems that this is not meet the SAFETY requirement arr_idx < arr.len(). Maybe some other thing need to be changed 🤔 .

pub fn get(&self, idx: usize) -> Option<T::Physical<'_>> {
        let (chunk_idx, arr_idx) = self.index_to_chunked_index(idx);
        let arr = self.downcast_get(chunk_idx)?;

        // SAFETY: if index_to_chunked_index returns a valid chunk_idx, we know
        // that arr_idx < arr.len().
        unsafe { arr.get_unchecked(arr_idx) }
    }

[orlp]

@reswqa Ah, the problem is that index_to_chunked_index has this fast-path: https://github.com/pola-rs/polars/blob/main/crates/polars-core/src/chunked_array/ops/downcast.rs#L100

        if self.chunks.len() == 1 {
            return (0, index);
        }

If the index is not found in chunk 0 it should return chunk 1, so this fast path is incorrect.

[reswqa]

@orlp, good catch, this should be the culprit. Will fix this.

[orlp]

@reswqa I'm curious, could you then remove the check from this again? I believe it should've just worked. If it's still broken just leave the check in place.

[reswqa]

@orlp Sure, I'm on the way.