In Tech Speack, NULL is a age old concept of nothing. This is not true in case of oracle. Oracle treats NULL as character value of length of 0.
So, the default data type of NULL is a character data type and to prove it, we create a view on a null column with an alias a, and then describe it to see the datatype and length of the string. Here is the code.....
create view myview
select null a from dual;
The describe command shows that the column a has a data type of a varchar2(0).