Under MySQL, $wgDBserver = ''; is assumed to be localhost, but in PostgresSQL, you get an error. I believe this is due to mysqli->real_connect() (https://secure.php.net/manual/en/mysqli.real-connect.php) interpreting the empty string as localhost, but no such thing for postgres. We should probably emulate that in MediaWiki.
This is a problem because the command line installer will write an empty db server to LocalSettings.php if no --dbserver is provided. The fact that this works on MySQL and not Postgres is problematic.