attempt to concatenate local 'tor_remote_addr' (a nil value)

[varuzam]

stack traceback:
coroutine 0:
        /var/www/src/nginx_anti_ddos_challenge.lua: in function </var/www/src/nginx_anti_ddos_challenge.lua:1>```

### Versions:

- Browser(s): not matters
- Nginx version: 1.14.2
- Operating system of web server running Nginx: Debian 10

### Nginx config:

access_by_lua_file /var/www/src/nginx_anti_ddos_challenge.lua;

[varuzam]

nginx error log

lua entry thread aborted: runtime error: /var/www/src/nginx_anti_ddos_challenge.lua:2736: attempt to concatenate local 'tor_remote_addr' (a nil value) stack traceback: coroutine 0: /var/www/src/nginx_anti_ddos_challenge.lua: in function </var/www/src/nginx_anti_ddos_challenge.lua:1>,

[varuzam]

the error happens when User-Agent is empty