Use $node->getLink() === null instead of is_null($node->getLink()) for consistency with modern PHP practices and better readability.

Co-authored-by: Copilot <[email protected]>

Author: Grotax

src/FeedIo/Rule/PublicId.php

@@ -20,7 +20,7 @@ public function setProperty(NodeInterface $node, \DOMElement $element): void
         $node->setPublicId($element->nodeValue);
         if ($element->nodeName === 'guid'
         && $element->getAttribute('isPermaLink') === 'true'
-        && is_null($node->getLink())) {
+        && $node->getLink() === null) {
             $node->setLink($element->nodeValue);
         }
     }