Do not try to compute promotion quantity for lines without type line

Closes #12351
(grafted from e4c4fcb90528908ed5135c1e78d8c7334d7c8c63)

--HG--
branch : 6.0

modules/sale_promotion/sale.py

@@ -241,6 +241,8 @@ class Promotion(
         if self.unit:
             quantity = 0
             for line in sale.lines:
+                if line.type != 'line':
+                    continue
                 if self.is_valid_sale_line(line):
                     quantity += Uom.compute_qty(line.unit, line.quantity,
                         self.unit)