[Config] fixed an edge case

src/Symfony/Component/Config/Tests/Util/XmlUtilsTest.php

@@ -104,6 +104,7 @@ class XmlUtilsTest extends \PHPUnit_Framework_TestCase
     public function getDataForPhpize()
     {
         return array(
+            array('', ''),
             array(null, 'null'),
             array(true, 'true'),
             array(false, 'false'),

src/Symfony/Component/Config/Util/XmlUtils.php

@@ -187,7 +187,7 @@ class XmlUtils
                 $cast = intval($value);
 
                 return '0' == $value[0] ? octdec($value) : (((string) $raw == (string) $cast) ? $cast : $raw);
-            case '-' === $value[0] && ctype_digit(substr($value, 1)):
+            case isset($value[1]) && '-' === $value[0] && ctype_digit(substr($value, 1)):
                 $raw = $value;
                 $cast = intval($value);