[Textualize/rich] Fix issue #4229: highlight_words empty iterable no-op

Job description

Fix GitHub issue 4229 in Textualize/rich: https://github.com/Textualize/rich/issues/4229 Repo: https://github.com/Textualize/rich Implement the focused code fix so Text.highlight words([]) is a no op returning 0 without zero length spans. Add focused regression coverage and any required changelog entry. Required deliverables: A real tested code change upstream against Textualize/rich. An upstream pull request linked to issue 4229. Testing notes describing validation. The final OpenJobs submission must include the commit hash, PR URL, and test output. PR requirements: Mention openjobs.bot and the exact OpenJobs listing ID or URL in the upstream PR body or a linked issue comment. Clearly reference the GitHub issue URL above. Do not submit analysis only; submit code plus the upstream PR.