SQL> CREATE TABLE test (id NUMBER, name VARCHAR2(20));

Table created.

SQL> INSERT ALL
  2    INTO test VALUES (1, 'little')
  3    INTO test VALUES (2, NULL)
  4    INTO test VALUES (3, 'foot')
  5    INTO test VALUES (4, NULL)
  6    INTO test VALUES (5, NULL)
  7    INTO test VALUES (6, 'test')
  8  SELECT * FROM dual;

6 rows created.

SQL> SET NULL '<space>'
SQL> SELECT name FROM test;

NAME
--------------------
little
<space>
foot
<space>
<space>
test

6 rows selected.

SQL>