[Sync EN] grapheme_strlen : Document return null on failure (#5568)

[github-actions]

Commit: https://github.com/php/doc-en/commit/e7d502fecc2e84d4c57e6451b16f68bec1295ba8
PR: https://github.com/php/doc-en/pull/5568

Fichiers FR à mettre à jour

• reference/intl/grapheme/grapheme-strlen.xml