[FIX] website_sale: don't handle pl promo code if already applied

When a pl promo code is applied, the code is shown in the promo code input.

Before this commit, if the user clicked on "Apply" (so with the same code) it
would remove the pl as it would go in the `if` condition that is supposed to
handle empty code modo to reset pl.

Coming from #34633

closes odoo/odoo#34734

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>