[Sync EN] Document that Locale::parseLocale() uses the default locale on empty string (#5851)

[github-actions]

Commit: https://github.com/php/doc-en/commit/815ee7695d8cd08a613bd94b9e52eae6692f77a8
PR: https://github.com/php/doc-en/pull/5851

Fichiers FR à mettre à jour

• reference/intl/locale/parse-locale.xml