Fixed non existant category with products issue

Author: ldusan84

app/code/Magento/CatalogGraphQl/Model/Resolver/CategoryTree.php

@@ -72,7 +72,7 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
 
         $rootCategoryId = $this->getCategoryId($args);
         $categoriesTree = $this->categoryTree->getTree($info, $rootCategoryId);
-        if (!empty($categoriesTree)) {
+        if (!empty($categoriesTree) && ($categoriesTree->count() > 0)) {
             $result = $this->extractDataFromCategoryTree->execute($categoriesTree);
             return current($result);
         } else {

dev/tests/api-functional/testsuite/Magento/GraphQl/Catalog/CategoryTest.php

@@ -112,6 +112,21 @@ public function testCategoriesTree()
         );
     }
 
+    public function testNonExistentCategoryWithProductCount()
+    {
+        $query = <<<QUERY
+{
+  category(id: 99) {
+      product_count
+    }
+}
+QUERY;
+
+        $response = $this->graphQlQuery($query);
+        $expectedResponse = ['category' => null];
+        $this->assertEquals($expectedResponse, $response);
+    }
+
     /**
      * @magentoApiDataFixture Magento/Catalog/_files/categories.php
      * @SuppressWarnings(PHPMD.ExcessiveMethodLength)