For arrays of aggregates, accessing a nonexistent value results in an error:

   a[2] <<< 2; @avg(a[3])

This is intentional, since @avg is considered undefined for a zero-element
list.  However, for scalars, we don't enforce this same thing:

   @avg(b) ; @max(b)

may spit out a generic division-by-zero error or a sentinel "0" value.
It should be made consistent with the vector-of-aggregates case, so that
only @count() is valid on an empty aggregate scalar.