Postgresql – How to dump PostgreSQL database whose owner has no password

First, some background on my setup:

The server has root access disabled. So, I log-in as (say) john who also belongs to the sudo group and is therefore able to run superuser commands.

I created a new password-less user^#1 santa (using the command sudo adduser --shell /bin/bash --gecos 'Santa Claus' --disabled-password santa).

Then I changed the login session's owner to santa using the command: sudo su - santa and created a new PostgreSQL database: createdb myapp_db (the database got created without asking me for password).

Now, given the condition, how do I make an SQL dump of the database myapp_db (whose owner is santa) using the command pg_dump?

This is supposed to work, but it isn't:

john@host:~$ pg_dump myapp_db -U santa -h localhost --no-owner -W > myapp_db_backup.sql
Password: 

When I run the above command, it asks me for "password" as you can see. What password am I supposed to enter here? I didn't enter any password for the database when creating it, nor does the database owner santa have any.

So, I tried entering the password of john, the sudo user, and got this error:

"FATAL: password authentication failed for user "santa""

I also tried (which didn't work either):

john@host:~$ su - santa
santa@host:~$ pg_dump myapp_db -U santa -h localhost --no-owner -W > myapp_db_backup.sql
Password:

This time, I created a password for the user santa and entered it. Still get the error:

"FATAL: password authentication failed for user "santa""

What am I missing here?

(Please let me know if I am missing any necessary details.)

More Information

(As requested in the comments.)

john@host:~$ sudo su postgres
postgres@host:~$ psql -l

                                         List of databases
          Name           |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges   
-------------------------+----------+----------+-------------+-------------+-----------------------
 app_db                  | santa    | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 postgres                | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 template0               | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
                         |          |          |             |             | postgres=CTc/postgres
 template1               | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
                         |          |          |             |             | postgres=CTc/postgres
(4 rows)

Output of \du santa:

john@host:~$ sudo -u postgres psql
postgres=# \du santa

                             List of roles
 Role name |                   Attributes                   | Member of 
-----------+------------------------------------------------+-----------
 santa     | Superuser, Create role, Create DB, Replication | {}