Reset amount to pay with Decimal

Otherwise amount to pay can not copy sign from line.

Closes #13023
(grafted from 30926e7b8b6fe5deaffeea6a9c8efb120ff2b638)

--HG--
branch : 6.0

modules/account_statement/statement.py

@@ -349,7 +349,7 @@ class Statement(Workflow, ModelSQL, ModelView):
                         new_line.invoice = None
                         line_offset += 1
                         lines.insert(index + line_offset, new_line)
-                        invoice_id2amount_to_pay[line.invoice.id] = 0
+                        invoice_id2amount_to_pay[line.invoice.id] = Decimal(0)
                         line.amount = amount_to_pay.copy_sign(line.amount)
                     else:
                         invoice_id2amount_to_pay[line.invoice.id] = (