[FIX] sale: T&C issue when changing partner

Steps to reproduce:
1. Create an invoice with a customer and enter some text in Terms and Conditions (field: comment).
2. Save the invoice.
3. Edit the invoice and change the customer.

Bug:
The text in Terms and Conditions was empty.

Same behavior as on the SO, if the company doesn't change the T&C should be kept.

Fine tuning of this commit: 9bf1f97d9b
Closes #32309

opw:1830838

closes odoo/odoo#33383

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

addons/sale/models/account_invoice.py

@@ -41,7 +41,7 @@ class AccountInvoice(models.Model):
         self.partner_shipping_id = addr and addr.get('delivery')
         if self.env.context.get('type', 'out_invoice') == 'out_invoice':
             company = self.company_id or self.env.user.company_id
-            self.comment = company.with_context(lang=self.partner_id.lang).sale_note
+            self.comment = company.with_context(lang=self.partner_id.lang).sale_note or (self._origin.company_id == company and self.comment)
 
     @api.multi
     def action_invoice_paid(self):