Description of problem:
On a sales order with a Delivery term with "Sales tax address" = Delivery the Sales tax group is always inherited from the delivery address and updated on the sales order even if on the address Sales tax group is blank. The expected behavior is that if the sales tax group on the delivery address is blank the value has not to be updated on the sales order (this exact behavior is the one experienced with purchase orders).
Repro steps:
1. Term of delivery with Sales tax address = delivery
2. Customer with 2 delivery address:
a. With sales tax group linked to it
b. Without sales tax group
3. Customer with a default Sales tax group and the created Delivery term
4. Create a Sales order with delivery address a, sales tax is inherited from delivery address
5. Change delivery address to b, Sales tax is updated and sales order even if delivery address has no Sales tax group