Handle rating when null is returned

[eby]

Deprecated function: round(): Passing null to parameter #1 ($num) of type int|float is deprecated in Drupal\arborcat\Controller\DefaultController->bibrecord_page()

Where:

arborcat/src/Controller/DefaultController.php

Line 168 in 6d82707

$ratings->average = round($ratings->average, 1);

Example bib: https://aadl.org/catalog/record/10610568