[FIX] point_of_sale: prevent validating pending electronic payments orders

Before this commit:

 1. Open a POS session configured with any terminal payment method
 2. Click on Payment, pick the terminal payment method
 3. Enter an amount
 4. Click on "Send" to send the amount to the terminal
 5. Click on "Validate" before the terminal returns anything
=>
The ticket is validated at $0.
The actual amount is credited in the terminal but the cash closing does
not contain this actual amount.

After this commit:
An error message appear if trying to validate such an order.
The validate button is also grayed out (but can still be clicked).
The pot file have also been updated

Note: the issue will also happen in version 14.0, but as I am a bit
afraid of the side effects that it might have (angry customers used to
the previous way), I prefer to push it first to the customer version
only. If I don't have echo of angry customers on this, I will do a
backward port and validate the forward ports.

opw-3292442

closes odoo/odoo#122539

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>