SQL> declare
  2    col clob;
  3  begin
  4    if col is null then
  5       dbms_output.put_line('col is null');
  6    else
  7       dbms_output.put_line('col is not null');
  8    end if;
  9
 10    col := 'abcd';
 11
 12    if col is null then
 13       dbms_output.put_line('#2 - col is null');
 14    else
 15       dbms_output.put_Line('#2 - col is not null');
 16    end if;
 17  end;
 18  /
col is null
#2 - col is not null

PL/SQL procedure successfully completed.

SQL>