[FIX] point_of_sale: correctly compute cash profit for closing popup

Current behavior:
If you add cash at the opening of a session that already has cash in the
cash register; the closing popup will show a negative profit (loss) for
the cash payment method.

Steps to reproduce:
- Open a POS session
- Add cash to the cash register (50€)
- Close the session
- Open a new session
- The popup show that there is already 50€ in the cash register, add
  cash to the cash register (100€)
- Close the session, the closing popup will show a loss of 50€ for the
  cash payment method. Wich is not true, it's a profit of 50€.

opw-3101297

closes odoo/odoo#108737

Signed-off-by: Heinz Robin (rhe) <rhe@odoo.com>