[FIX] website: avoid traceback during edition of menu missing URL

Before this commit if a menu had been saved without URL, a string +
boolean concatenation error could happen when updating the menu.

After this commit the False boolean is replaced by an empty string to
avoid the problem.

task-2781260

closes odoo/odoo#89147

X-original-commit: 0288e0e91a071c7ec4b8adb0474d82a46dea9c9c
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/website/models/website_menu.py

@@ -184,7 +184,7 @@ class Menu(models.Model):
             menu_id = self.browse(menu['id'])
             # if the url match a website.page, set the m2o relation
             # except if the menu url is '#', meaning it will be used as a menu container, most likely for a dropdown
-            if menu['url'] == '#':
+            if not menu['url'] or menu['url'] == '#':
                 if menu_id.page_id:
                     menu_id.page_id = None
             else: