Nginx Map how to check value if empty

[c0nw0nk]

So I have the following Map

map $http_cf_connecting_ip $client_ip_from_cf {
default $http_cf_connecting_ip;
}

How can I make it so if the client did not send that $http_ header it makes
$client_ip_from_cf variable value = $binary_remote_addr

Not sure how to check in a map if that http header is present.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,272744,272744#msg-272744