Fixes parallel case with empty list

[elijahbenizzy]

This was broken as we assumed the list was not empty. This special-cases it. We should probably be breaking this into functions so we can add more unit tests, but given the experimental nature of this I'm not worried. Don't want to over-engineer. The fix will work nicely for now.

Changes

How I tested this

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

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