tls: do not hang without `newSession` handler

When listening for client hello parser events (like OCSP requests), do
not hang if `newSession` event handler is not present.

Fix: https://github.com/joyent/node/issues/8660
Fix: https://github.com/joyent/node/issues/25735



Reviewed-By: Fedor Indutny <fedor@indutny.com>
PR-URL: https://github.com/joyent/node/pull/25739