Fix typo in Query::getAbortMessage()

We currently have the following code for \Drupal\search_api\Query\Query::getAbortMessage():

  public function getAbortMessage() {
    return is_bool($this->aborted) ? $this->aborted : NULL;
  }

$this->aborted can either be NULL, or TRUE or an error message (string or translatable markup). But, as you can see, instead of returning only an error message or NULL, we return only TRUE or NULL.