Don't allow an empty Subject when creating a Certificate

Misconfiguration (e.g. an empty policy section in the config file) can
lead to an empty Subject. Since certificates should have unique Subjects
this should not be allowed.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5114)

(cherry picked from commit e505f1e8)