metatag-8.x-1.x-dev/src/Plugin/Field/FieldFormatter/MetatagEmptyFormatter.php

<?php

namespace Drupal\metatag\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\Attribute\FieldFormatter;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Plugin implementation of the 'metatag_empty_formatter' formatter.
 */
#[FieldFormatter(
  id: 'metatag_empty_formatter',
  label: new TranslatableMarkup('Empty formatter'),
  field_types: [
    'metatag',
  ]
)]
class MetatagEmptyFormatter extends FormatterBase {

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    // Does not actually output anything.
    return [];
  }

}