SQL> create table clob_tab (id number, data clob);

Table created.

SQL> create or replace trigger tr_clob_01
  2  after insert on clob_tab
  3  for each row
  4  declare
  5   clb clob;
  6   data_new varchar2(100);
  7   amnt binary_integer := 100;
  8  begin
  9   clb := :new.data;
 10   DBMS_LOB.READ (clb,amnt,1,data_new);
 11   dbms_output.put_line(data_new);
 12  exception 
 13   when no_data_found then
 14      dbms_output.put_line('No data found');
 15  end;
 16  /

Trigger created.

SQL> insert into clob_tab values(1,'ABC');
ABC

1 row created.

SQL> insert into clob_tab values(2,empty_clob());
No data found

1 row created.

SQL> create or replace trigger tr_clob_01
  2  after insert on clob_tab
  3  for each row
  4  declare
  5   clb clob;
  6  begin
  7   clb := :new.data;
  8   if clb is null then
  9    dbms_output.put_line('CLOB pointer is NULL');
 10   else
 11    dbms_output.put_line('CLOB pointer is NOT NULL');
 12   end if;
 13  end;
 14  /

Trigger created.

SQL> insert into clob_tab values(1,'ABC');
CLOB pointer is NOT NULL

1 row created.

SQL> insert into clob_tab values(2,empty_clob());
CLOB pointer is NOT NULL

1 row created.

SQL> insert into clob_tab values(3,null);
CLOB pointer is NULL

1 row created.

SQL> declare
  2   clb clob;
  3   new_data varchar2(10) := '0123456789';
  4  begin
  5   insert into clob_tab values(1,empty_clob()) returning data into clb;
  6   dbms_lob.write(clb,10,1,new_data);
  7   commit;
  8  end;
  9  /
CLOB length is 0

PL/SQL procedure successfully completed.

SQL> select * from clob_tab;

        ID
----------
DATA
--------------------------------------------------------------------------------
         1
0123456789