If only there was a site to ask user-oriented questions about desktop and laptop software that would provide a common answer to exist for a long time ...
> By default, that is, if no password has been set up, the stored password is null and password authentication will always fail for that user
Yeah, there you go.
If you don't want to use password authentication, you can always set up Ident authentication (which it tries to use by default anyway, I think). Then it essentially just checks if the system and postgres username match (or you can specify a mapping).
There's also a file you can put in ~/ with the password in it so that it always has the password when you try to connect.