[Nginx] returns 403 for requests with UA null

[Nginx] returns 403 for requests with UA null

Nginx blocking blank user agent.
Sometime apps 'backgroundrequest always visit a url, and these requests' user agent is black, so I want block them.

Nginx configuration for a location

Location/{if ($ http_user_agent = "") {access_log off; return 403;} default_type text/html; content_by_lua 'ngx. say ("

Simulate request

Lzz @ ubuntu: conf $ curl 'HTTP: // localhost' 

lzz@ubuntu:conf$ curl -H "User-Agent:" 'http://localhost'