how to handle no data found exception when the function return date ?

How to handle no data found exception when the function return date and there is no data found

i handled it with sysdate but i want to retrieve nothing when no data found not display the sysdate

here is the code

CREATE OR REPLACE FUNCTION XX_GET_TRX_DATE_RCV_FUNCTION

(P_ORG IN number,P_N_ITEM IN  number,P_PO_NO IN number)

RETURN DATE

IS

vb_trx_date DATE;

     

BEGIN

  

------------------ Description -------------------------

-- This function returns the qty recieved DATE from a specific PO

--------------------------------------------------------     

    

if P_PO_NO is null

then return null;

else

    select TRANSACTION_DATE