It seems that MySQL's COALESCE allows to input NULL values as if all the values are NULL, NULL will be returned.

I quote from MySQL Manual:

COALESCE(list)
    Returns first non-NULL element in list:

mysql> SELECT COALESCE(NULL,1);
        -> 1
mysql> SELECT COALESCE(NULL,NULL,NULL);
        -> NULL

UPDATE: ops, i wasn't careful enough to check the solution offered. In that case NULL won't be returned so Jenda's assumption is right.