Return error when invalid currentPage is provided

Adds missing validation for the minimum page value

Fixes #727

Author: pmclain

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

@@ -66,6 +66,10 @@ public function resolve(
             ]
         ];
         $searchCriteria = $this->searchCriteriaBuilder->build($field->getName(), $args);
+        if ($args['currentPage'] < 1) {
+            throw new GraphQlInputException(__('currentPage value must be greater than 0.'));
+        }
+
         $searchCriteria->setCurrentPage($args['currentPage']);
         $searchCriteria->setPageSize($args['pageSize']);
         $searchResult = $this->filterQuery->getResult($searchCriteria, $info);

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

@@ -71,6 +71,10 @@ public function resolve(
         array $args = null
     ) {
         $searchCriteria = $this->searchCriteriaBuilder->build($field->getName(), $args);
+        if ($args['currentPage'] < 1) {
+            throw new GraphQlInputException(__('currentPage value must be greater than 0.'));
+        }
+
         $searchCriteria->setCurrentPage($args['currentPage']);
         $searchCriteria->setPageSize($args['pageSize']);
         if (!isset($args['search']) && !isset($args['filter'])) {

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

@@ -1131,6 +1131,37 @@ public function testFilterProductsThatAreOutOfStockWithConfigSettings()
         $this->assertEquals(1, $response['products']['total_count']);
     }
 
+    /**
+     * Verify that invalid page numbers return an error
+     *
+     * @magentoApiDataFixture Magento/Catalog/_files/products_with_layered_navigation_attribute.php
+     * @expectedException \Exception
+     * @expectedExceptionMessage currentPage value must be greater than 0
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
+     */
+    public function testInvalidPageNumbers()
+    {
+        $query = <<<QUERY
+{
+  products (
+    filter: {
+      sku: {
+        like:"simple%"
+      }
+    }
+    pageSize: 4
+    currentPage: 0
+  ) {
+    items {
+      sku
+    }
+  }
+}
+QUERY;
+
+        $this->graphQlQuery($query);
+    }
+
     /**
      * Asserts the different fields of items returned after search query is executed
      *