Boolean without default declared

If a column with a boolean datatype doesn't have a default value. What
type of value is set if nothing is inserted into that column? Here is
my test table and the queries I have tried. I can't seem to be able to
select the rows where happy has no value.

Table "public.users"
Column | Type | Modifiers
--------+-----------------------+-----------
id | character varying(32) |
email | text |
happy | boolean |
money | numeric |

*select * from users; *
id | email | happy | money
----+--------+-------+---------
4 | me | | 1324.23
4 | me | | 1324.23
3 | as | | 123.2
1 | afjssd | t |

*select * from users where happy;*
id | email | happy | money
----+--------+-------+-------
1 | afjssd | t |

*select * from users where not happy;*
id | email | happy | money
----+-------+-------+-------
(0 rows)

*select * from users where happy = NULL;*
id | email | happy | money
----+-------+-------+-------
(0 rows)

*select * from users where happy = '';*
ERROR: invalid input syntax for type boolean: ""