SQL> drop table t;

Table dropped.

SQL> create table t (v date);

Table created.

SQL>  insert into t values (null);

1 row created.

SQL> insert into t values (to_date('13/11/1956','DD/MM/YYYY'));

1 row created.

SQL> commit;

Commit complete.

SQL> select 'Date is: '||nvl(v, sysdate) r from t;
R
----------------------------
Date is: 14/01/2013 09:18:48
Date is: 13/11/1956 00:00:00

2 rows selected.

SQL> select 'Date is: '||to_char(nvl(v, sysdate),'DD-MON-YYYY') r from t;
R
--------------------
Date is: 14-JAN-2013
Date is: 13-NOV-1956

SQL> select 'Date is: '||nvl(to_char(v,'DD-MON-YYYY'),'Unknown') r from t;
R
--------------------
Date is: Unknown
Date is: 13-NOV-1956