You seem to treat NULLs as if they have a specific meaning. So then the question is this: what should count(NULL) be?

Here are some options:

1) count(NULL) = 0, so apparently NULL is just like an empty set at least some of the time. This means that some of the code will treat NULLs as empty sets and other code will not, leaving the burden on the programmer to keep in mind these implicit differences.

2) count(NULL) = 1 because NULL is a value, albeit a sentinel value. This can lead to tricky problems where a count() suggests that there's some data, while, in fact, there is none.

3) count(NULL) = NULL. If the idea behind this option is that sentinel value cannot be operated on, then this is pretty much like throwing an error at every NULL, which, in turn, will result in the necessity to guard many expressions with some error (NULL) handling code.

One of the things to note is that an empty set has rather unambiguous semantics, while a NULL presents options each of which can be justified depending on how a particular person thinks about this special value. The line between "a value has not yet been assigned", "no value has been assigned" and "a value has been unassigned" is very blurry. On the other hand, the line between "there is no value" and "there is a value" is pretty clear.