Protect: no empty headers (#14859)

* Protect

We should never send an empty header to the Protect server.
It can result in false positives for misconfigured servers.

* Update modules/protect.php

Co-Authored-By: Jeremy Herve <jeremy@jeremy.hu>

Co-authored-by: Jeremy Herve <jeremy@tagada.hu>

modules/protect.php

@@ -407,7 +407,7 @@ function get_headers() {
 		);
 
 		foreach ( $ip_related_headers as $header ) {
-			if ( isset( $_SERVER[ $header ] ) ) {
+			if ( ! empty( $_SERVER[ $header ] ) ) {
 				$output[ $header ] = $_SERVER[ $header ];
 			}
 		}