[nginx] Return 403 for request with empty UA

nginx blocking blank user agent .
sometimes apps’ backgroud request always visit a url, and these requests’ user agent is black, so I want block them.

nginx configuration for one location

<code>       location / {
            <span>if</span> (<span>$http_user_agent</span> = <span>""</span>){
                access_log off;
                <span>return</span><span>403</span>;
            }

            default_<span>type</span>   text/html;
            content_by_lua <span>'
                ngx.say("<h1>你好 baby </h1>")
            '</span>;
        }</code>

Simulated request

<code>lzz<span>@ubuntu</span><span>:conf</span><span>$ </span>curl <span>'http://localhost'</span>
<h1>你好 baby <<span>/h1></code>

<code>lzz@ubuntu:conf$ curl -H "User-Agent:" 'http://localhost'
<span><<span>html</span>></span><span><<span>head</span>></span><span><<span>title</span>></span>403 Forbidden<span></<span>title</span>></span><span></<span>head</span>></span><span><<span>body</span><span>bgcolor</span>=<span>"white"</span>></span><span><<span>center</span>></span><span><<span>h1</span>></span>403 Forbidden<span></<span>h1</span>></span><span></<span>center</span>></span><span><<span>hr</span>></span><span><<span>center</span>></span>ngx_openresty/1.2.6.1<span></<span>center</span>></span><span></<span>body</span>></span><span></<span>html</span>></span></code>

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