Nginx configuration to redirect requests for non-existent files to an external URI without using if.

nginx.conf

# Redirect all requests for non-existent files to another web
# site, used on my own site to redirect people to my GitHub
# account page.
server {
  server_name example.com;
  location / {
    try_files $uri $uri/index.html @external;
  }
  location @external {
    rewrite ^ $scheme://anotherexample.com$request_uri redirect;
  }
}