Fix reCAPTCHA v3 validation when token is in $_POST['captcha']

Civi/Formprotection/Recaptcha.php

@@ -144,13 +144,13 @@ class Recaptcha {
       return;
     }
 
-    if (isset($_POST['g-recaptcha-token'])) {
+    if (!empty($_POST['g-recaptcha-token'])) {
       $token = $_POST['g-recaptcha-token'];
     }
-    else if (isset($_POST['g-recaptcha-response'])) {
+    else if (!empty($_POST['g-recaptcha-response'])) {
       $token = $_POST['g-recaptcha-response'];
     }
-    else if (isset($_POST['captcha'])) {
+    else if (!empty($_POST['captcha'])) {
       $token = $_POST['captcha'];
     }