getAssertion: Skip attStmt if attestationFormatsPreference is not set

[robin-nitrokey]

If attestationFormatsPreference is absent or a single element list of the string "none", omit attestation from the output.

https://fidoalliance.org/specs/fido-v2.2-rd-20230321/fido-client-to-authenticator-protocol-v2.2-rd-20230321.html#sctn-getAssert-authnr-alg

The current implementation defaults to packed instead (as for makeCredential):

fido-authenticator/src/ctap2.rs

Lines 1676 to 1677 in 0fdecc9

	let att_stmt_fmt =
	SupportedAttestationFormat::select(data.attestation_formats_preference.as_ref());

fido-authenticator/src/ctap2.rs

Lines 2019 to 2023 in 0fdecc9

	fn select(preference: Option<&AttestationFormatsPreference>) -> Option<Self> {
	let Some(preference) = preference else {
	// no preference, default to packed format
	return Some(Self::Packed);
	};