Do not test quantity for lines of type different than line

Closes #12274

modules/account_rule/purchase.py

@@ -10,5 +10,5 @@ class Line(metaclass=PoolMeta):
     def get_invoice_line(self):
         with Transaction().set_context(
                 taxes=[t.id for t in self.taxes],
-                return_=self.quantity < 0):
+                return_=self.type == 'line' and self.quantity < 0):
             return super().get_invoice_line()

modules/account_rule/sale.py

@@ -10,5 +10,5 @@ class Line(metaclass=PoolMeta):
     def get_invoice_line(self):
         with Transaction().set_context(
                 taxes=[t.id for t in self.taxes],
-                return_=self.quantity < 0):
+                return_=self.type == 'line' and self.quantity < 0):
             return super().get_invoice_line()

modules/account_rule/tests/scenario_account_rule.rst

@@ -95,6 +95,9 @@ Test rules with a sale::
     >>> line = sale.lines.new()
     >>> line.product = product
     >>> line.quantity = -1
+    >>> line = sale.lines.new()
+    >>> line.type = 'comment'
+    >>> line.description = 'Sample'
     >>> sale.click('quote')
     >>> sale.click('confirm')
     >>> sale.state