[FIX] point_of_sale: fix decimals converted into units after parse

In a POS session, in a language that does not use dot (.) as decimal separator (i.e. French),
when adding a Product, it can happen that the decimal part of the price is not converted
correctly after applying "field_utils.parse.float". (i.e. 2.69 becomes 269)
The issue happens in "set_unit_price" function.

To prevent this to happen, "field_utils.parse.float" will only be applied if the price given
as parameter is not a number.

opw-2357848

closes odoo/odoo#60294

Signed-off-by: Anh Thao PHAM <kitan191@users.noreply.github.com>