concat(NULL,NULL) returns empty string

Hi,

The documentation of the function concat states: Concatenate all arguments.
NULL arguments are ignored.
Intuitively I would expect that

concat(NULL, NULL) IS NULL

but this is not the case as it equals the empty string. As no empty string
was passed, the empty string seems
to come from nowhere. I found this behavior not immediately clear from the
documentation..

Kind regards,

Ingmar Brouns