https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21Plugin%21Field%21FieldType%21StringItemBase.php/8.9.x
StringItemBase.php | Drupal API
phpdrupalapi
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21Plugin%21Field%21FieldType%21StringItemBase.php/function/StringItemBase%3A%3AisEmpty/10
function StringItemBase::isEmpty | Drupal API
functionisemptydrupalapi