B2B-2464: Improve category children loading

- Flattener only is concerned with custom_attributes
- This fix is to prevent getChildren from being needlessly called via buildOutputDataArray (note that children is reset to empty array after that call)

Author: danmooney2

app/code/Magento/CatalogGraphQl/Model/Category/Hydrator.php

@@ -57,11 +57,13 @@ public function __construct(
      */
     public function hydrateCategory(Category $category, $basicFieldsOnly = false) : array
     {
-        if ($basicFieldsOnly) {
-            $categoryData = $category->getData();
-        } else {
-            $categoryData = $this->dataObjectProcessor->buildOutputDataArray($category, CategoryInterface::class);
+        if (!$basicFieldsOnly) {
+            // initialize custom attributes (to be consequently included in subsequent getData call below)
+            $category->getCustomAttributes();
         }
+
+        $categoryData = $category->getData();
+
         $categoryData['id'] = $category->getId();
         $categoryData['uid'] = $this->uidEncoder->encode((string) $category->getId());
         $categoryData['children'] = [];