[FIX] website_sale: Display pricelist price if no price_unit

Steps to reproduce:

  - Install website_sale module
  - Enable discount and advanced pricelist in settings
  - Create product with sale price 0$ and set a website in
    eCommerce + publish the product
  - Create pricelist PPP with Discount Policy as
    "Show public price & discount to the customer" and selectable in the website
  - Go to the product and set an extra price of 10$ for the new pricelist
  - Go to the product in the eshop and select the pricelist PPP
  - Add the product to the shop cart

Issue:

  The price displayed is 0$ instead of 10$.

Cause:

  Since price_unit equal 0$, not possible to calculate the discount and
  therefore using the 0$ value.

Solution:

  Use price of pricelist in case 'discount_policy' is 'without_discount'
  and price_unit equal 0$.

opw-2652192

closes odoo/odoo#78570

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