Bug 31144: When modifying an order we should not load the vendor default discount

Bug 31144: When modifying an order we should not load the vendor default discount

After placing an order in a basket with a discount of "0" if you attempt to modify we will load the vendor default discount, this can lead to accidentally applying a discount when modifying other order parts

To test:
0 - Have a vendor with a discount set
1 - Add an order to a basket
2 - While adding, set discount to 0
3 - Add order - prices are calculated ocrrectly
4 - Modify order
5 - Note discount is populated with vendor discount
6 - Cancel
7 - Apply patch
8 - Modify order
9 - Discount remains at 0
10 - Repeat with a vendor with no discount set
11 - Placing order not specifiying discount it remains blank
12 - Modifying order discount remains blank
13 - Modify order and set discount
14 - Confirm it is retained on modifcation

Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>