Taxonomy terms are created with a parent 0

Problem/Motivation

When creating a new term, the parent field references a taxonomy term with ID 0 (which doesn't exist). It's also saved to the database like this.

\Drupal\taxonomy\TermStorage::create() apparently does this, for undocumented reasons.

/**
 * {@inheritdoc}
 *
 * @param array $values
 *   An array of values to set, keyed by property name. A value for the
 *   vocabulary ID ('vid') is required.
 */
public function create(array $values = []) {
  // Save new terms with no parents by default.
  if (empty($values['parent'])) {
    $values['parent'] = [0];
  }
  $entity = parent::create($values);
  return $entity;
}

Steps to reproduce

$term = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->create(['vid' => 'tags']);
$term->get('parent')->isEmpty(); // returns FALSE
$term->get('parent')->getValue(); // returns [['target_id' => 0]]

Proposed resolution

If there's no reason for doing this, let's remove this code.
If there is a reason for doing this, I think it's appropriate to document this (preferably in the code).

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet