Fall back to stub entity when entity doesn't exist during param converting

Fall back to creating stub entity instead of just returning the plain UUID string when entity doesn't exist during param converting. This would simplify the logic a lot in each resource plugin.

So \Drupal\relaxed\ParamConverter\DbConverter::convert() \Drupal\relaxed\ParamConverter\DocIdConverter::convert() needs to change, so that we don't need all is_string() checks in the resource plugins.

Because things like this loose typing just doesn't make any sense:

  /**
   * @param string | \Drupal\multiversion\Entity\WorkspaceInterface $workspace
   * @param string | \Drupal\Core\Entity\ContentEntityInterface $existing_entity
   * @param \Drupal\Core\Entity\ContentEntityInterface $received_entity
   *
   * @return \Drupal\rest\ResourceResponse
   */
  public function put($workspace, $existing_entity, ContentEntityInterface $received_entity) {
    if (is_string($workspace)) {
      throw new NotFoundHttpException();
    }
    // [...]