Vulnerability Instance

Twig has unguarded calls to `__isset()` and to array-accesses when the sandbox is enabled
### Description

In a sandbox, and 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.**

### Resolution

The sandbox mode now ensures access to array-like's properties is allowed.

The patch for this issue is available [here](https://github.com/twigphp/Twig/commit/ec39a9dccc5fb4eaaba55e5d79a6f84a8dd8b69d) for the 3.11.x branch, and [here](https://github.com/twigphp/Twig/commit/b957e5a44cc0075d04ccff52f8fa9d8e6db3e3a0) for the 3.x branch.

### Credits

We would like to thank Jamie Schouten for reporting the issue and Nicolas Grekas for providing the fix.