[3.11] [3.12] gh-109413: libregrtest: enable mypy's `--strict-optional` check on most files (GH-112586) (GH-112602) (#112603)

[3.12] gh-109413: libregrtest: enable mypy's `--strict-optional` check on most files (GH-112586) (GH-112602)

gh-109413: libregrtest: enable mypy's `--strict-optional` check on most files (GH-112586)

(cherry picked from commit 36dbebed44785579034ffb1a59f0a1a5b987c9bf)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>