Fix TypeScript errors in stats tests: avoid passing undefined for exactOptionalPropertyTypes

Agent-Logs-Url: https://github.com/githubnext/tsessebe/sessions/4c38da0f-a871-4aa4-bf5d-7133398e72bd

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot