[FIX] website_sale: restore possibility to remove promo code pricelist

Since 5c9cea4e, it was not possible to remove an applied promo code
pricelist by adding an empty promo code on checkout (eg removing the one shown
in the promo code input).

Indeed, when sending an empty promo code, the `search()` done in the controller
would not find any pricelist as promo would be en empty string. For the rest,
check code on mentionned commit.

See `sale_get_order()` method docstring about `code` param: "If empty, it's a
special case to reset the pricelist with the first available else the default.".

Fixes #34633