Connect to "postgres" when creating database

Otherwise, PostgreSQL will try to connect to a database with the same
name as the user. If this database does not exist, pgsql will fail with
"<<"database \"some_user\" does not exist">>,[]}}". A simple solution
is to connect to the postgres database instead, which should always
exist.

(cherry picked from commit 23cfe50bc2b229ff2fba94f8dbf3ec3bf2bd0129)