on_change methods must not return anyting

issue4667
review8081002

account.py

@@ -50,7 +50,7 @@ class InvoiceLine:
 
     @fields.depends('origin')
     def on_change_product(self):
-        return super(InvoiceLine, self).on_change_product()
+        super(InvoiceLine, self).on_change_product()
 
     @fields.depends('origin')
     def on_change_account(self):

purchase.py

@@ -42,4 +42,4 @@ class PurchaseLine:
     @fields.depends('_parent_purchase.warehouse',
         '_parent_purchase.invoice_address')
     def on_change_product(self):
-        return super(PurchaseLine, self).on_change_product()
+        super(PurchaseLine, self).on_change_product()

sale.py

@@ -48,4 +48,4 @@ class SaleLine:
 
     @fields.depends('_parent_sale.warehouse', '_parent_sale.shipment_address')
     def on_change_product(self):
-        return super(SaleLine, self).on_change_product()
+        super(SaleLine, self).on_change_product()