None attestation format accepts non-empty attStmt

[ScottHelme]

Problem

The None attestation format handler in None::validateAttestation() returns true unconditionally without checking the contents of attStmt.

Per W3C WebAuthn spec §8.7 (None Attestation Statement Format):

"If attStmt is a properly formed attestation statement, verify that attStmt is an empty CBOR map."

Currently, an attacker could send an attestation object with fmt: "none" but include arbitrary data in attStmt (e.g. {"foo": "bar"}) and it would be silently accepted.

Suggested fix

Add a check at the top of None::validateAttestation() that verifies attStmt is empty:

if (\count($this->_attestationObject['attStmt']) > 0) {
    throw new WebAuthnException('invalid none attestation: attStmt must be empty', WebAuthnException::INVALID_DATA);
}