TemplateString: Define parameter type as string|null for $for par…

…ameter of `parseTemplates()` method

src/TemplateString.php

@@ -49,7 +49,7 @@ public function __construct($format, $args = null)
     /**
      * Parse template strings
      *
-     * @param null $for template name
+     * @param  ?string $for template name
      * @return HtmlDocument
      * @throws Exception in case of missing template argument or unbounded open or close templates
      */