Spec 1.03 Part 6 section 5.2.7 States "A union with no fields present has the same meaning as a NULL value." Also applies for Optional values in Structures.

The specification does not make a difference between the terms 'not present' i.e. "no value" and value set as 'NULL'. Some types have valid null encodings while some don't. This is important when making implementations because normally it would not make sense to have both conditions 'not-present' and 'null', but with the current spec both are valid and different values. For example consider the following cases.

Case 1. Union switch value encoded as 0 i.e. 'Null'
Case 2. Union switch value encoded as non-0 and having a value that is by itself 'NULL' e.g. a String with length of -1

Question 1. Is the situation in Case 2. allowed by the specification?
Question 2. If the answer of Question 1. is yes, is there a difference between the two i.e. is it allowed to encode the situation in Case 2. as being the same as in 1.