declare 
  v_bool boolean;
  v_out  varchar2(10);
begin
  v_bool := ('' is null);
--  v_bool := (null = null);
  if v_bool = true then
    dbms_output.put_line('true');
  elsif v_bool = false then
    dbms_output.put_line('false');    
  else
    dbms_output.put_line('null');     
  end if;
  
  v_out := 
    case v_bool
      when true  then 'true'
      when false then 'false'
      when null  then 'null'
     -- else 'grosses Problem'
    end;
  
  dbms_output.put_line(v_out);
  
end;