fix: list.mean fast path shouldn't produce NaN

[reswqa]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #15652 will not alter performance

_{Comparing reswqa:list_mean (b705e17) with main (99ab9c0)}

Summary

✅ 22 untouched benchmarks

[orlp]

Could you write this entire thing without unsafe? It feels unnecessary.

if let Some(slice) = values.get(current_offset as usize..*end as usize) {
    if slice.len() == 0 {
        None
    } else {
        sum_slice::<_, S>(slice) / NumCast::from(slice.len()).unwrap(),
    }
} else {
    None
}

[orlp]

Oh I didn't update my browser before commenting, I'll do it myself as a chore.