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