Add a test for an empty NextProto message

Add a test for an empty NextProto message

It is valid according to the spec for a NextProto message to have no
protocols listed in it. The OpenSSL implementation however does not allow
us to create such a message. In order to check that we work as expected
when communicating with a client that does generate such messages we have
to use a TLSProxy test.

Follow on from CVE-2024-5535

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24718)