Issue #3529265: Fix empty plural translations being incorrectly treated as valid due to delimiter

This merge request addresses Issue #3529265: formatPlural() may return an empty string when a .po file contains plural translations with both msgstr[0] and msgstr[1] left empty.

Although these are effectively "empty" translations, Drupal stores them using the plural delimiter character (\x03, defined as PoItem::DELIMITER), which causes the system to wrongly treat the translation as non-empty. This results in formatPlural() rendering an empty string instead of falling back to the original source string.