POSIX os.putenv forbids null bytes in env values, so
monkeypatch.setenv(SOUP_EDIT_GOVERNOR_DB, 'x\x00.db') raised
ValueError at setenv time on ubuntu/macos before the code under
test ran (windows tolerated it). Assert _validate_governor_db_override
rejects the null byte directly; the validated-None fallback branch is
already covered cross-platform by test_env_override_out_of_bounds_falls_back.