DMN14 — The result of product([]) is not defined

The product() function may take a list but it is not defined what the return value is when the list is empty.

It could be zero, or could be null. Other list function return null like `median( [ ] ) = null`, and `stddev( [ ] ) = null`, so it seems reasonable to make `product([]) = null`, but likely more business friendly to make `product([]) = 0`. I'd go for the latte.

Define result of product([]) to be null

As discussed on the RTF meeting held 2021-03-23 for consensus (kept consistency with other functions)