[FIX] point_of_sale: Cash opening difference not shown in closing popup

Current behavior:
When there is an opening difference, the closing popup does not show it.

Steps to reproduce:
- Create a new POS session
- Add some cash to the register (100€)
- Close the session with the same amount
- Open the session again
- Change the cash amount in the register (80€)
- When closing the session the popup should show the -20€ difference

opw-3101297

closes odoo/odoo#113012

X-original-commit: fef0a13114e4eebb294c8c8008ab0a7c9dfab864
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>