You could try this

this config snippet is assuming thet you've already got the TLS cert/pem file for lemmy.world elsewhere in your nginx.config

http {
  server {
    listen 80;
    listen 443 ssl;
    server_name lemmy.world;

    if ($scheme = "http") {
        return 307 https://$host$request_uri;
    }

    location / {
        proxy_pass http://lemmy-ui:1234;
        proxy_set_header Host $host;
    }
}

If you get redirected to lemmy.world:1234, then add absolute_redirect off; in the 'server' block

Last thing - 307 is a temporary redirect, you might to change it to a permanent one once you've confirmed it's working as intended