Do not apply negative unit price after promotion

We must just set a unit price of 0.

issue10903
review377421002
(grafted from 0f2befb01e316a15a38ccd4edfbc33671c35cee0)

--HG--
branch : 6.0

sale.py

@@ -298,6 +298,8 @@ class Promotion(
                 continue
             context = self.get_context_formula(line)
             new_price = self.get_unit_price(**context)
+            if new_price < 0:
+                new_price = Decimal(0)
             if line.unit_price >= new_price:
                 line.unit_price = round_price(new_price)
                 line.promotion = self