[FIX] website: safely access menu recordset

Introduced with 7fb016f9966b
[0] access could crash, recordset could be empty after `filtered`.

Step to reproduce:
  - Add admin group to website top level menu (Website > Debug > Menu)
  - Try to access frontend as non admin

opw-2573763
task-2574346

closes odoo/odoo#72535

X-original-commit: 759d920d799d332220d86467d83aeb9334b61ce2
Signed-off-by: Romain Derie <rdeodoo@users.noreply.github.com>

addons/website/models/website.py

@@ -159,9 +159,12 @@ class Website(models.Model):
                 # don't add child menu if parent is forbidden
                 if menu.parent_id and menu.parent_id in menus:
                     menu.parent_id._cache['child_id'] += (menu.id,)
+
             # prefetch every website.page and ir.ui.view at once
             menus.mapped('is_visible')
-            website.menu_id = menus and menus.filtered(lambda m: not m.parent_id)[0].id or False
+
+            top_menus = menus.filtered(lambda m: not m.parent_id)
+            website.menu_id = top_menus and top_menus[0].id or False
 
     # self.env.uid for ir.rule groups on menu
     @tools.ormcache('self.env.uid', 'self.id')