/**
 * Implements hook_options_list_alter().
 */
function {{ machine_name }}_options_list_alter(array &$options, array $context) {
  // Check if this is the field we want to change.
  if ($context['fieldDefinition']->id() == 'field_option') {
    // Change the label of the empty option.
    $options['_none'] = t('== Empty ==');
  }
}