fix for system xml.

Author: a.chorniy

app/code/Magento/Config/Model/Config/Structure.php

@@ -398,7 +398,7 @@ private function getFieldsRecursively(array $elements = [])
                     $this->getFieldsRecursively($element['children'])
                 );
             } else {
-                if ($element['_elementType'] === 'field' && isset($element['label'])) {
+                if ($element['_elementType'] === 'field') {
                     $structurePath = (isset($element['path']) ? $element['path'] . '/' : '') . $element['id'];
                     $configPath = isset($element['config_path']) ? $element['config_path'] : $structurePath;