fix: adding new items to an order should not crash

[johannes-vogel]

No description provided.

[vl-leon]

@johannes-vogel Does this happen because the order is not valid and the order item references a product that does not exist?

[johannes-vogel]

It's a new OrderItem that's added to an existing Order.

[vl-leon]

I am trying to identify the use case. The variable before is not initialized because the product referenced by this OrderItem does not exist, right?

[johannes-vogel]

For not yet existing Order Items, the quantity of the before image is undefined. This leads to the orderChanged event being emitted with 12 - undefined (which is NaN).

The correct change is to default to zero instead of the early exit.