[FIX] account: soften the constraint on company_id on partner's fp

Have a partner with company_id = False
Your user is currently in company A

Before this commit, only FPs with no company id could be set on the partner

After this commit, FP without company or belonging to company A are available

The drawback is that since this computation takes place in an onchange, when you first arrive on the partner
no filter is applied and you can choose whatever FP that appear
This might cause access errors when it is misued

related to #24256
closes #24507

addons/account/models/partner.py

@@ -394,8 +394,7 @@ class ResPartner(models.Model):
         required=True)
     property_account_position_id = fields.Many2one('account.fiscal.position', company_dependent=True,
         string="Fiscal Position",
-        help="The fiscal position will determine taxes and accounts used for the partner.", oldname="property_account_position",
-        domain="[('company_id', 'in', [company_id, False])]")
+        help="The fiscal position will determine taxes and accounts used for the partner.", oldname="property_account_position")
     property_payment_term_id = fields.Many2one('account.payment.term', company_dependent=True,
         string='Customer Payment Terms',
         help="This payment term will be used instead of the default one for sales orders and customer invoices", oldname="property_payment_term")
@@ -445,5 +444,9 @@ class ResPartner(models.Model):
 
     @api.onchange('company_id')
     def _onchange_company_id(self):
+        company = self.env['res.company']
         if self.company_id:
-            return {'domain': {'property_account_position_id': [('company_id', 'in', [self.company_id.id, False])]}}
+            company = self.company_id
+        else:
+            company = self.env.user.company_id
+        return {'domain': {'property_account_position_id': [('company_id', 'in', [company.id, False])]}}