Since [this commit] when a user opens a dropup menu, a traceback is
raised.

Steps to reproduce the bug:
- Go to website
- Enter in edit mode
- Change the header template to "Sidebar"
- Save
- Open the "Mitchell Admin" menu (a dropup menu)

=> A traceback is displayed.

The problem was that we have code to handle the orientation of the
opening of dropdowns menu but this code is also called when we open a
dropup menu and it has not been designed for this case. This commit
fixes the issue by adapting the code to be able to handle dropup menus.

[this commit]: https://github.com/odoo/odoo/commit/b6963dc21bad4b012aace16b9ea9264c33a0d618



opw-3371946

closes odoo/odoo#126085

Signed-off-by: Romain Derie (rde) <rde@odoo.com>