sqlplus statements for NULL values

The following oracle sql statement selects 1/1/2011 as dob when it is null:
SELECT name, NVL(dob,'1-JAN-2011') FROM Employee;
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The following oracle sql statement selects 10 when quantity is null.

SELECT item_name, NVL(quantity, 10) From stock;
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When the field value is in NUMBER, when it is null, i want to display the string "NULL". How do it in sqlplus?
select nvl( to_char(age), 'NULL' ) age
2 from emp where staff_no=2356;
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How to insert null value in field in sqlplus?

INSERT INTO emp VALUES ('shivaji', 250, null);
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