Only debug auth_cookie_malformed when non-empty

login-debug.php

@@ -9,7 +9,9 @@
 **/
 
 function debug_login_auth_cookie_malformed($cookie, $scheme) {
-    error_log("LOGIN: auth_cookie_malformed: scheme={$scheme} cookie={$cookie}");
+    if ($cookie) {
+        error_log("LOGIN: auth_cookie_malformed: scheme={$scheme} cookie={$cookie}");
+    }
 }
 add_action('auth_cookie_malformed', 'debug_login_auth_cookie_malformed', 10, 2);