ENGCOM-4008: Fixed non existant category with products issue #311

 - Merge Pull Request magento/graphql-ce#311 from ldusan84/graphql-ce:feature/category-with-products-fix
 - Merged commits:
   1. d41c407
   2. 5bf7364
   3. 8d828da
   4. 996ba90

Author: magento-engcom-team

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

@@ -11,6 +11,7 @@
 use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
 use Magento\Framework\GraphQl\Config\Element\Field;
 use Magento\Framework\GraphQl\Exception\GraphQlInputException;
+use Magento\Framework\GraphQl\Exception\GraphQlNoSuchEntityException;
 use Magento\Framework\GraphQl\Query\ResolverInterface;
 
 /**
@@ -72,11 +73,12 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
 
         $rootCategoryId = $this->getCategoryId($args);
         $categoriesTree = $this->categoryTree->getTree($info, $rootCategoryId);
-        if (!empty($categoriesTree)) {
-            $result = $this->extractDataFromCategoryTree->execute($categoriesTree);
-            return current($result);
-        } else {
-            return null;
+
+        if (empty($categoriesTree) || ($categoriesTree->count() == 0)) {
+            throw new GraphQlNoSuchEntityException(__('Category doesn\'t exist'));
         }
+
+        $result = $this->extractDataFromCategoryTree->execute($categoriesTree);
+        return current($result);
     }
 }

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

@@ -10,11 +10,13 @@
 use Magento\Catalog\Api\Data\CategoryInterface;
 use Magento\Catalog\Model\ResourceModel\Category\Collection as CategoryCollection;
 use Magento\Framework\DataObject;
+use Magento\TestFramework\TestCase\GraphQl\ResponseContainsErrorsException;
 use Magento\TestFramework\TestCase\GraphQlAbstract;
 use Magento\Catalog\Api\Data\ProductInterface;
 use Magento\Catalog\Api\ProductRepositoryInterface;
 use Magento\TestFramework\ObjectManager;
 
+
 class CategoryTest extends GraphQlAbstract
 {
     /**
@@ -146,6 +148,21 @@ public function testGetCategoryById()
         );
     }
 
+    public function testNonExistentCategoryWithProductCount()
+    {
+        $query = <<<QUERY
+{
+  category(id: 99) {
+      product_count
+    }
+}
+QUERY;
+
+        $this->expectException(ResponseContainsErrorsException::class);
+        $this->expectExceptionMessage('GraphQL response contains errors: Category doesn\'t exist');
+        $this->graphQlQuery($query);
+    }
+
     /**
      * @magentoApiDataFixture Magento/Catalog/_files/categories.php
      * @SuppressWarnings(PHPMD.ExcessiveMethodLength)