bug: Server12::new no longer rejects empty certificate at construction

[HMBSbige]

Server::new only validates the private key and no longer rejects an empty certificate. Dtls::new_12(config, DtlsCertificate { certificate: vec![], .. }, now) constructs successfully, deferring the misconfiguration from startup to handshake time. CryptoContext::new used to catch this at construction.

dimpl/src/dtls12/server.rs

Lines 127 to 143 in cda235a

	pub fn new(config: Arc<Config>, certificate: crate::DtlsCertificate, now: Instant) -> Server {
	// unwrap: malformed private_key bytes are a programmer error from the
	// caller who constructed DtlsCertificate; panic matches the prior
	// CryptoContext::new behavior which also panicked on empty/invalid
	// key material.
	let private_key = config
	.crypto_provider()
	.key_provider
	.load_private_key(&certificate.private_key)
	.expect("Failed to parse server private key");
	let auth = AuthMode::Certificate {
	certificate: certificate.certificate,
	private_key,
	};
	let engine = Engine::new(config, auth);
	Self::new_with_engine(engine, now)
	}