When you set a column of a table to use a default value, do you have to make that column not null?

I defaulted a column of a table to be 'BOY'. The data dictionary shows that the column is defaulted but when I checked from that column, I can still see some nulls values indicating that the default is not being used.

ALTER TABLE EMPLOYEE MODIFY (EMP_TYPE DEFAULT 'BOY');

select column_name, data_default, table_name from dba_tab_columns where owner in ('DANIEL') and table_name in ('EMPLOYEE');

COLUMN_NAME DATA_DEFAULT TABLE_NAME
---------------------- -------------------- -------------------------
EMP_TYPE 'BOY' EMPLOYEE



SQL> select EMP_TYPE from EMPLOYEE;

EMP_TYPE
--------










MAN

EMP_TYPE
--------
MAN

12 rows selected.