[Sync EN] docs(locale.getKeywords): clarify return values and empty string argument (#5844)

[github-actions]

Commit: https://github.com/php/doc-en/commit/d8de774ae203bccf3ffe02895d8c8fc754377451
PR: https://github.com/php/doc-en/pull/5844

Archivos ES a actualizar

• reference/intl/locale/get-keywords.xml