[FIX] sale: _get_conversion_rate called with no date

Steps to reproduce the bug:
- Define two pricelists P1(€) the default one and P2($) with discount_policy = 'without_discount'
- Activate P1 and P2 on the website
- Go on the shop and select P2

Bug:

An error was raised from function _get_conversion_rate because the arguments date and company_id were not defined
because the function _get_real_price_currency was called from _website_product_id_change with
the model "self.env['sale.order.line'] and not with a recordset. So the fields self.company_id and self.order_id.date_order
were not defined.

opw:1969849

closes odoo/odoo#32726

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>