Add Commerce Product SKU validation.

This will require the patch at https://www.drupal.org/node/2319719 for this to work.
By casting product_id to an integer (like the user constraint does), we could have avoided the if ($product_id).
See that code:
    $value_taken = (bool) \Drupal::entityQuery('user')
      // The UID could be NULL, so we cast it to 0 in that case.
      ->condition('uid', (int) $uid, '<>')