gistfile1.nginxconf

    # Replace blank variables with -
    #
    # map $foo $foo_h {
    #     default $foo;
    #     "" -;
    # }
    map $host $host_h { default $host; "" -; }
    log_format lighttpd
        '$remote_addr $host_h $remote_user [$time_local] "$request" $status '
        '$body_bytes_sent "$http_referer" "$http_user_agent"';
    access_log /var/log/nginx/access.log lighttpd;

gistfile2.nginxconf

    # This version is a little more readable, but I haven't tested it.

    # Replace blank $host with -
    map $host $host_or_hyphen { 
        default $host; 
        "" -;  
    }
    log_format lighttpd
        '$remote_addr $host_or_hyphen $remote_user [$time_local] "$request" '
        '$status $body_bytes_sent "$http_referer" "$http_user_agent"';
    access_log /var/log/nginx/access.log lighttpd;