Function does not return, but gives error..

I have the following function to determine wether or not a user is 
member of a group, however I have a small problem with it:
a group without members results in groupres being NULL (I have checked 
this), however
IF groupres = NULL
THEN
...
END IF;
is not trapped... I have tried to use array_upper(groupres,1) < 1 OR 
array_upper(groupres,1) = NULL
yet, I get no message about it... It is just that I find this strange 
behaviour, I could find a way to work around this with the if before the 
loop:

Anyone any idea?

TIA,
Michiel
--- function is_in_group(name,name) ---
CREATE OR REPLACE FUNCTION "public"."is_in_group" (name, name) RETURNS 
boolean AS
$body$
DECLARE      userid    INTEGER;      groupres  INTEGER[];      username  ALIAS FOR $1;      groupname ALIAS FOR $2;
BEGIN    SELECT INTO userid usesysid FROM pg_user WHERE usename = $1;
    IF NOT FOUND    THEN        RETURN false; -- not a known user, so the user is not a member 
of the group    END IF;       SELECT INTO groupres grolist FROM pg_group WHERE groname = $2;       IF NOT FOUND    THEN
      RAISE WARNING 'Unknown group ''%''', $2;        RETURN false;    END IF;       IF groupres = NULL    THEN
--no members in the group, so this user is not member either        RAISE WARNING 'Group ''%'' has no members.', $2;
   RETURN false;    END IF;    RAISE WARNING 'Groupres: %',groupres;       IF array_lower(groupres,1) >= 1    THEN
  FOR currentgroup IN 
 
array_lower(groupres,1)..array_upper(groupres,1) LOOP              IF groupres[currentgroup] = userid              THEN
                RETURN true;              END IF;         END LOOP;    END IF;
 
    -- if we can get here, the user was not found in the group    -- so we return false       RETURN false;
END;
$body$
LANGUAGE 'plpgsql' STABLE CALLED ON NULL INPUT SECURITY INVOKER;
--- end function ---