Unguarded calls to __isset() and to array-accesses when the sandbox is enabled in Twig

Twig is a template language for PHP. In a sandbox, an attacker can access attributes of Array-like objects as they were not checked by the security policy. They are now checked via the property policy and the `__isset()` method is now called after the security check. This is a BC break. This issue has been patched in versions 3.11.2 and 3.14.1. All users are advised to upgrade. There are no known workarounds for this issue.