[FIX] point_of_sale: Parse quantity like a float

Steps to reproduce:

  - Install "Point of sale" and "l10n_fr" modules
  - Switch to french Company
  - Go to user preference and change language to "French"
  - Create a new POS with french certification installed
  - Run the POS
  - Try to change quantity of a product

Issue:

  Impossible to encode a quantity with decimals (add a line at 0 instead)

Cause:

  The popup return value is a string.
  The value is used before coverting it to float.

Solution:

  Parse quantity value like a float before using it.

opw-2608707

closes odoo/odoo#74303

Signed-off-by: bon-odoo <nboulif@users.noreply.github.com>