Ensure certificate callbacks work correctly in TLSv1.3

The is_tls13_capable() function should not return 0 if no certificates
are configured directly because a certificate callback is present.

Fixes #7140

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7141)