When the Server receives a "Write" request with Structure where some optional field is absent, what is the server supposed to do?

a) Assume some default value and perform the write with it
b) skip the writing of the part that is absent
c) undefined

The "Write" here is an example, similar questions arise with input arguments to Call, and also data returned to the Client (or contained in PubSub datasets).

The same issue may apply to empty Unions (Union with Switch Field equal to 0), but there in the mappings the spec says "A Union with no fields present has the same meaning as a NULL value. A Union with any field present is not a NULL value even if the value of the field itself is NULL.", so that it at least some answer, it looks like that in both cases the Server is actually supposed to process the value somehow?