[FIX] point_of_sale: fix decimal point

Before this fix there was no check if there was already a decimal point
in the value when using the onscreen keypad.

Fixes #32034

Steps to reproduce:

* Open up POS interface
* Add an item
* Press the decimal point button multiple times (2 or more)
* Note that pressing additional number keys do not display
* Backspace must be pressed for the number of digits entered,
  plus the multiple decimal points, to clear
  all the hidden characters in the value buffer
* Same behavior occurs with the Discount and Price
  values

Current behavior:
Multiple decimal point characters can be
"entered" and are inserted into value buffer
(Qty, Disc, or Price), but do not display on
screen.
Additional digits entered after multiple decimal
point presses are inserted into buffer, but do
not display on screen.
Additional digits entered after the last valid
digit on the screen are also inserted into the
buffer.
Backspace must be used to remove the invisible
characters from the value buffer.

Expected behavior:
The value buffer for Qty, Disc, or Price should
more closely match what is displayed on the
screen.
Characters which are invalid and aren't
displayed on the screen, should not be inserted
into the value buffer, including the decimal
point button.

closes odoo/odoo#32089

Signed-off-by: Quentin Lejeune (qle) <qle@odoo.com>