Updated Shipping and Discount payload formats

— Date values of "discountCoupon" in the Discount payload were changed to the following format to be like all our dates in other APIs: "2021-08-10 21:00:00 +0000"
— Certain null values in both payloads were replace with empty values or arrays. Example of old values:

{ “options”:null, “wholesalePrices”:null, “favorites”:null,... }

New values:

{ “options”:[], wholesalePrices”:[], “favorites”:{ “count”:0,“displayedCount”:“0"}