Do not ignore empty associated data with AES-SIV mode

Do not ignore empty associated data with AES-SIV mode

The AES-SIV mode allows for multiple associated data items
authenticated separately with any of these being 0 length.

The provided implementation ignores such empty associated data
which is incorrect in regards to the RFC 5297 and is also
a security issue because such empty associated data then become
unauthenticated if an application expects to authenticate them.

Fixes CVE-2023-2975

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21384)