Node::preSaveRevision

public Node::preSaveRevision(EntityStorageInterface $storage, \stdClass $record)

public function preSaveRevision(EntityStorageInterface $storage, \stdClass $record) {
  parent::preSaveRevision($storage, $record);

  if (!$this->isNewRevision() && isset($this->original) && (!isset($record->revision_log) || $record->revision_log === '')) {
    // If we are updating an existing node without adding a new revision, we
    // need to make sure $entity->revision_log is reset whenever it is empty.
    // Therefore, this code allows us to avoid clobbering an existing log
    // entry with an empty one.
    $record->revision_log = $this->original->revision_log->value;
  }
}