don't log an nginx warning for every authenticated request (#24682)

Since 0.18.2, every request that went through /auth logged an uninitialized "go2rtc_stream_proxy" variable warning, because only the go2rtc stream locations set that variable. The warning is now turned off in the /auth location, and the header value sent to /auth is unchanged.

docker/main/rootfs/usr/local/nginx/conf/auth_location.conf

@@ -5,6 +5,8 @@ location /auth {
     ## Essential Proxy Configuration
     internal;
     proxy_pass $upstream_auth;
+    # $go2rtc_stream_proxy is only set by the go2rtc stream locations
+    uninitialized_variable_warn off;
 
     ## Headers