Fix extend implement interface in Parser

This is part of the update to allow interfaces to implement interfaces.
A single extend statement to add an implementation of an interface
without field declarations is valid. This was caught by tests and brings
in a change from graphql/graphql-js#2084

Author: Kingdutch

src/Language/Parser.php

@@ -1627,7 +1627,9 @@ private function parseInterfaceTypeExtension() : InterfaceTypeExtensionNode
         $interfaces = $this->parseImplementsInterfaces();
         $directives = $this->parseDirectives(true);
         $fields     = $this->parseFieldsDefinition();
-        if (count($directives) === 0 &&
+        if (
+            count($interfaces) === 0 &&
+            count($directives) === 0 &&
             count($fields) === 0
         ) {
             throw $this->unexpected();
@@ -1636,7 +1638,7 @@ private function parseInterfaceTypeExtension() : InterfaceTypeExtensionNode
         return new InterfaceTypeExtensionNode([
             'name'       => $name,
             'directives' => $directives,
-            'interfaces'  => $interfaces,
+            'interfaces' => $interfaces,
             'fields'     => $fields,
             'loc'        => $this->loc($start),
         ]);