HHVM results in a fatal error when accessing a property that contains a NUL byte.

**Describe the bug**

HHVM results in a fatal error when accessing a property which name contains a NUL byte.

This is most likey inherited from the fact that PHP 5.0 prefixed private/protected properties with NUL byte, later on in PHP 7.0, PHP started throwing an Error when attempting to access a property using NUL byte, which HHVM also did.

However, since 09/12/2020, this changed to a fatal error: https://github.com/facebook/hhvm/commit/68ecce841e8897d1ec2af89c3f2600cfea1aabc4

**Standalone code, or other way to reproduce the problem**

https://gist.github.com/fredemmott/a39231a07477138e4b502c8932b977aa

**Expected behavior**

undefined property warning/exception (depending on hhvm config)

**Actual behavior**

fatal error.

Start with the standalone reproduction in the linked gist and inspect the behavior change introduced by commit 68ecce841e8897d1ec2af89c3f2600cfea1aabc4. Confirm that accessing a property containing a NUL byte produces the expected undefined-property warning or exception rather than a fatal error, including the relevant HHVM configuration behavior.

Written by the indexing model from the issue text.