MC-21811: Canonical_url displays the backend domain instead of relative

- add return null

Author: cpartica

app/code/Magento/CatalogGraphQl/Model/Resolver/Category/CanonicalUrl.php

@@ -54,5 +54,6 @@ public function resolve(
             $baseUrl = $category->getUrlInstance()->getBaseUrl();
             return str_replace($baseUrl, '', $category->getUrl());
         }
+        return null;
     }
 }

app/code/Magento/CatalogGraphQl/Model/Resolver/Product/CanonicalUrl.php

@@ -53,5 +53,6 @@ public function resolve(
             $product->getUrlModel()->getUrl($product, ['_ignore_category' => true]);
             return $product->getRequestPath();
         }
+        return null;
     }
 }