feat(postgres): use standard Unix socket directory if host is omitted

[izebit]

Does your PR solve an issue?

fixes #3969

Based on the postgresql doc, if host part is skipped in the url, default socket dir is used.

Is this a breaking change?

No, it isn’t a breaking change. It extends the connection URL format to allow URLs without a host, which were previously considered invalid and made to_url_lossy panicked.