Do not lookup zero-length session ID

A condition was removed by commit 1053a6e2

; presumably it was an
unintended change. Restore the previous behavior so the get_session_cb
won't be called with zero-length session ID.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/4236)