concat(NULL,NULL) returns empty string

[Ingmar Brouns <swingi@xxxxxxxxx>]

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