nightly-2024-10-22: fix: Allow array map on empty arrays (#6305)

# Description

## Problem\*

Resolves a private issue sent to me on slack

## Summary\*

Array map is a fairly old method written before we had
`std::mem::zeroed`. Now that we have zeroed, we can allow mapping empty
arrays since we can use `zeroed` to get the filler `U` value for the
starting elements.

## Additional Context

While I was at it I included a small clarification to `reduce`'s docs
that it requires a non-empty array

## Documentation\*

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.