GraphQl-727: currentPage: 0, currentPage: 1 and currentPage: -1 produces the same output for products query when filtering is used

- added pageSize validation;
- fixed code style issues.

Author: lenaorobei

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

@@ -69,6 +69,9 @@ public function resolve(
         if ($args['currentPage'] < 1) {
             throw new GraphQlInputException(__('currentPage value must be greater than 0.'));
         }
+        if ($args['pageSize'] < 1) {
+            throw new GraphQlInputException(__('pageSize value must be greater than 0.'));
+        }
 
         $searchCriteria->setCurrentPage($args['currentPage']);
         $searchCriteria->setPageSize($args['pageSize']);

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

@@ -74,6 +74,9 @@ public function resolve(
         if ($args['currentPage'] < 1) {
             throw new GraphQlInputException(__('currentPage value must be greater than 0.'));
         }
+        if ($args['pageSize'] < 1) {
+            throw new GraphQlInputException(__('pageSize value must be greater than 0.'));
+        }
 
         $searchCriteria->setCurrentPage($args['currentPage']);
         $searchCriteria->setPageSize($args['pageSize']);

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

@@ -381,8 +381,10 @@ public function testSearchWithFilterWithPageSizeEqualTotalCount()
 }
 QUERY;
         $this->expectException(\Exception::class);
-        $this->expectExceptionMessage('GraphQL response contains errors: currentPage value 2 specified is greater ' .
-            'than the 1 page(s) available');
+        $this->expectExceptionMessage(
+            'GraphQL response contains errors: currentPage value 2 specified is greater ' .
+            'than the 1 page(s) available'
+        );
         $this->graphQlQuery($query);
     }
 
@@ -1043,8 +1045,10 @@ public function testQueryPageOutOfBoundException()
 QUERY;
 
         $this->expectException(\Exception::class);
-        $this->expectExceptionMessage('GraphQL response contains errors: currentPage value 2 specified is greater ' .
-            'than the 1 page(s) available.');
+        $this->expectExceptionMessage(
+            'GraphQL response contains errors: currentPage value 2 specified is greater ' .
+            'than the 1 page(s) available.'
+        );
         $this->graphQlQuery($query);
     }
 
@@ -1075,8 +1079,10 @@ public function testQueryWithNoSearchOrFilterArgumentException()
 QUERY;
 
         $this->expectException(\Exception::class);
-        $this->expectExceptionMessage('GraphQL response contains errors: \'search\' or \'filter\' input argument is ' .
-            'required.');
+        $this->expectExceptionMessage(
+            'GraphQL response contains errors: \'search\' or \'filter\' input argument is ' .
+            'required.'
+        );
         $this->graphQlQuery($query);
     }
 
@@ -1162,6 +1168,37 @@ public function testInvalidPageNumbers()
         $this->graphQlQuery($query);
     }
 
+    /**
+     * Verify that invalid page size returns an error
+     *
+     * @magentoApiDataFixture Magento/Catalog/_files/products_with_layered_navigation_attribute.php
+     * @expectedException \Exception
+     * @expectedExceptionMessage pageSize value must be greater than 0
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
+     */
+    public function testInvalidPageSize()
+    {
+        $query = <<<QUERY
+{
+  products (
+    filter: {
+      sku: {
+        like:"simple%"
+      }
+    }
+    pageSize: 0
+    currentPage: 1
+  ) {
+    items {
+      sku
+    }
+  }
+}
+QUERY;
+
+        $this->graphQlQuery($query);
+    }
+
     /**
      * Asserts the different fields of items returned after search query is executed
      *
@@ -1171,7 +1208,7 @@ public function testInvalidPageNumbers()
     private function assertProductItems(array $filteredProducts, array $actualResponse)
     {
         $productItemsInResponse = array_map(null, $actualResponse['products']['items'], $filteredProducts);
-
+        // phpcs:ignore Generic.CodeAnalysis.ForLoopWithTestFunctionCall
         for ($itemIndex = 0; $itemIndex < count($filteredProducts); $itemIndex++) {
             $this->assertNotEmpty($productItemsInResponse[$itemIndex]);
             $this->assertResponseFields(