Fix xml parser issue

Author: dvynograd

Diff for: lib/internal/Magento/Framework/Xml/Parser.php

@@ -111,7 +111,10 @@ protected function _xmlToArray($currentNode = false)
                         $value = ['_value' => $value, '_attribute' => $attributes];
                     }
                     if (isset($content[$node->nodeName])) {
-                        if (!isset($content[$node->nodeName][0]) || !is_array($content[$node->nodeName][0])) {
+                        if (
+                            (is_string($content[$node->nodeName]) || !isset($content[$node->nodeName][0]))
+                            || (is_array($value) && !is_array($content[$node->nodeName][0]))
+                        ) {
                             $oldValue = $content[$node->nodeName];
                             $content[$node->nodeName] = [];
                             $content[$node->nodeName][] = $oldValue;