[FIX] pos_sale_margin: correctly compute margin when no cost is set

Current behavior:
If there was no cost set on a product, the margin couldn't be calculated
for the PoS sale report.

Steps to reproduce:
- Install PoS and Sales
- Create a product with no cost
- Open a session and make an order with this item
- Close the session and go to the PoS sale report
- Add the margin measure to the report, and show the stats by product
- The new product does not have any margin.

opw-3120208

closes odoo/odoo#110429

X-original-commit: 9105f803095d7ddb07bc62ec1e2f2982215e273e
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>

addons/pos_sale_margin/report/sale_report.py

@@ -9,5 +9,5 @@ class SaleReport(models.Model):
 
     def _fill_pos_fields(self, additional_fields):
         values = super()._fill_pos_fields(additional_fields)
-        values['margin'] = 'SUM(l.price_subtotal - l.total_cost / CASE COALESCE(pos.currency_rate, 0) WHEN 0 THEN 1.0 ELSE pos.currency_rate END)'
+        values['margin'] = 'SUM(l.price_subtotal - COALESCE(l.total_cost,0) / CASE COALESCE(pos.currency_rate, 0) WHEN 0 THEN 1.0 ELSE pos.currency_rate END)'
         return values