Set qty to purchase to 0 when removed

[metas-lc]

Is this a bug or feature request?

bug
found white testing #1462

What is the current behavior?

when you remove qty to purchase in "create purchase orders", you get an error

Which are the steps to reproduce?

1. create a new sales order, add a line

2. make sure that product has bpartner product and that bpartner has a discount schema set for purchase side
   
   

3. select the line, go to actions, start "create purchase orders"

4. update Bestellte menge (qty to purchase) then remove it and press enter
   => error:
   Ihr Test hat einen bisher unentdeckten Fehler offengelegt.
   ↵Bitte leiten Sie diese Meldung an metas weiter:↵↵Assumption failure: Parameter qtyToPurchase is not null for JSONDocumentChangedEvent(operation=replace, path=qtyToPurchase, value=)"

What is the expected or desired behavior?

no errors, qty shall be set to 0 if you remove it

[metas-lc]

IT

1. create a new sales order, add a line
2. select the line, actions -> create purchase orders
3. set qty to purchase, remove the qty
4. press enter
   => qty becomes 0, no errors OK
5. remove the qty, press tab / select another field / select somewhere outside the fields
   => qty becomes 0, no errors OK