[FIX] website_sale: prevent adding empty fields

Steps to reproduce:
1- install eComerce app and activate debug mode
2- go to Website > Configuration > Websites
3- on a website product page extra fields tab add an empty line and save
4- visit any product page on the website (500: internal server error)

Bug:
adding an empty field crashes the website server on the product page

Fix:
made the field required

opw-2945621

closes odoo/odoo#100318

Signed-off-by: William Braeckman (wbr) <wbr@odoo.com>