theme_options_none() is replaced by hook_options_list_alter()

In Drupal 7 you could override theme_options_none() to change the label of the "empty" (no selection) option.

/**
 * Overrides theme_options_none().
 */
function MYTHEME_options_none($variables) {
  // Check if this is the field we want to change.
  if ($variables['instance']['field_name'] == 'field_option') {
    // Change the label of the empty option.
    return t('== Empty ==');
  }
}

In Drupal 8, this can be done by an alter hook.

/**
 * @param array $options
 *   The array of options for the field, as returned by hook_options_list(). An
 *   empty option (_none) might have been added, depending on the field
 *   properties.
 *
 * @param array $context
 *   An associative array containing:
 *   - field_definition: The field definition
 *     (\Drupal\Core\Field\FieldDefinitionInterface).
 *   - entity: The entity object the field is attached to
 *     (\Drupal\Core\Entity\EntityInterface).
 */
function hook_options_list_alter(array &$options, array $context) {
  // Check if this is the field we want to change.
  if ($context['field']->id() == 'node.article.field_option') {
    // Change the label of the empty option.
    $options['_none'] = t('== Empty ==');
  }
}