B2B-2469: Improve category children loading

- added very top root node dummy for proper tree handling

Author: rganin

app/code/Magento/CatalogGraphQl/Model/Resolver/Products/DataProvider/CategoryTree/Wrapper/Forgery.php

@@ -16,6 +16,11 @@
  */
 class Forgery
 {
+    /**
+     * Most top node id in the tree structure.
+     */
+    private const DUMMY_ROOT_ID = 0;
+
     /**
      * Flat index of the tree.
      *
@@ -50,9 +55,12 @@ function ($element) {
             },
             explode('/', $category->getPath())
         );
-        $parentId = null;
+        if (!$this->hasNodeById(self::DUMMY_ROOT_ID)) {
+            $this->indexById[self::DUMMY_ROOT_ID] = new Node(self::DUMMY_ROOT_ID, $this);
+        }
+        $parentId = self::DUMMY_ROOT_ID;
         foreach ($pathElements as $id) {
-            if ($parentId && $this->hasNodeById($parentId)) {
+            if ($this->hasNodeById($parentId)) {
                 if (!$this->hasNodeById($id)) {
                     $this->indexById[$id] = new Node($id, $this);
                     $this->getNodeById($parentId)->addChild($this->indexById[$id]);