Render <nolink> menu items as plain text instead of empty Markdown links

Problem/Motivation

When generating the llms.txt file from Drupal menus, parent items configured with the <nolink> route (often used as non-clickable containers for submenus) are currently rendered as Markdown links with an empty URL.

For example, given a menu structure where "Menu Item 1" and "Menu Item 2" are parent containers without links:

Current output: The module renders these items with empty parentheses ():

Expected output: Since these items are structural labels and do not point to any resource, they should be rendered as plain text in the Markdown list to provide a cleaner structure for LLMs, avoiding the syntax of broken or empty links.

## Main Menu
- Menu Item 1
  - [Menu Item A](http://example.com)
- Menu Item 2
  - [Menu Item B](http://example.com)
- [Menu Item C](http://example.com)

Proposed resolution

Update the menu tree rendering logic to check if a menu link is routed to <nolink>. If so, render the item title as a plain string instead of wrapping it in the []() Markdown link syntax.