[FIX] point_of_sale: 0 => +/- => backspace => 0 and not -

When writting number on the point of sale keypad, if you click on the
"+/-" button, you will get 0 (with buffer "-0"), but then you need to
backspace 2 times to remove "-".

This seems odd since this mean that:

=> "+/-" "backspace" "backspace" "5" will get 5.0 units
=> "+/-" "backspace" "backspace" "backspace" will remove line

instead of what would be expected:

=> "+/-" "backspace" "5" will get 5.0 units
=> "+/-" "backspace" "backspace" will remove the line

opw-2169157
closes #43495

closes odoo/odoo#43507

X-original-commit: d3489146
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>