[FIX] website: menus with null URL would throw an error on display

Before this commit, if a website.menu had a URL set to null, it would throw an
error when displaying that menu.
It is normally not possible to do so in v11 but after a migration, menus can be
empty. (eg: Using an empty menu to display a divider in a dropdown menu)

Now we return an empty url instead of an error

addons/website/models/website.py

@@ -815,7 +815,7 @@ class Menu(models.Model):
             url = self.page_id.sudo().url
         else:
             url = self.url
-            if not self.url.startswith('/'):
+            if url and not self.url.startswith('/'):
                 if '@' in self.url:
                     if not self.url.startswith('mailto'):
                         url = 'mailto:%s' % self.url