Ignore order items with a NULL unit price

Problem/Motivation

I'm experiencing crashes currently due to the core promotion BuyXGetY offer when used in combination with commerce_pricelist. When the GET order item is automatically added, if the promotion then no longer applies, on the next refresh, buildOrderLines() will get an order item that has a "NULL" unit price, with a quantity of zero (that is because commerce_pricelist doesn't know how to resolve a price for a quantity of 0...)

One alternative fix would be to skip order items with a quantity of zero... But I don't believe that's the right fix as we update the Klarna order after fulfilment and we do want order items with a quantity of 0 to be shown in the Klarna invoice.