Explicitly validate NULL values in the rating

On HelpfulnessBlock's validateForm():

if (!form_state->getValue('helpfulness_rating')) {
      $form_state->setErrorByName('helpfulness_rating', $this->t('Please indicate if this page is helpful or not.'));
}

Since when the rating is "No" the value is 0, it would fall on the error condition when it shouldn't. We should be explicit and check for a NULL value here instead.