count(col), unlike count(*), does not count rows for which col is NULL.
However, if col's data type is not a scalar (e.g. a collection, tuple,
or user-defined type) it behaves like count(*), counting NULLs too.
The cause is that get_dynamic_aggregate() converts count() to
the count(*) version. It works for scalars because get_dynamic_aggregate()
intentionally fails to match scalar arguments, and functions::get() then
matches the arguments against the pre-declared count functions.
As we can only pre-declare count(scalar) (there's an infinite number
of non-scalar types), we change the approach to be the same as min/max:
we make count() a generic function. In fact count(col) is much better
as a generic function, as it only examines its input to see if it is
NULL.
A unit test is added. It passes with Cassandra as well.
Fixes#14198.
Closes#14199
78f4ee385f