issue #2405 update smarty

and made Piwigo compatible with smarty 5.x

include/smarty/src/UndefinedVariable.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Smarty;
+
+/**
+ * class for undefined variable object
+ * This class defines an object for undefined variable handling
+ */
+class UndefinedVariable extends Variable {
+
+	/**
+	 * Always returns an empty string.
+	 *
+	 * @return string
+	 */
+	public function __toString() {
+		return '';
+	}
+}