PostgreSQL: Remove password requirement for user postgres
Whether a password is required or not has nothing to do with pg_shadow and whether a password is actually defined for the user. Yes, I know, that's weird.

pg_hba.conf controls the authentication method. If you want to request a password, use md5 authentication. If you want to allow login with no password to anyone, use trust. If you want to require the same username in the operating system as in PostgreSQL, use peer (UNIX, only for local connections) or sspi (Windows).

If there's a password set, but pg_hba.conf doesn't tell PostgreSQL to ask for it, the password is ignored.

If pg_hba.conf tells PostgreSQL to ask for a password but there's none set, then all login attempts will fail no matter what password is supplied.