Since #24, the plural expression may be undefined. This PR ensures that this is always a string (possibly empty) to avoid a deprecated notice in PHP 8.1.
PHP Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in /home/travis/build/polylang/dynamo/tmp/wordpress/wp-includes/pomo/plural-forms.php on line 101
Since #24, the plural expression may be undefined. This PR ensures that this is always a string (possibly empty) to avoid a deprecated notice in PHP 8.1.