[captcha] if there is no captcha enabled, default to allowing things instead of denying them

Author: shish

core/Util/Captcha.php

@@ -31,6 +31,10 @@ public static function check(?string $bypass_if = null): bool
             return true;
         }
 
-        return (bool)send_event(new CheckCaptchaEvent())->passed;
+        $passed = send_event(new CheckCaptchaEvent())->passed;
+        if ($passed === null) {
+            $passed = true;
+        }
+        return $passed;
     }
 }