crypto/x509: always emit a critical SAN extension if the Subject is empty.

crypto/x509: always emit a critical SAN extension if the Subject is empty.

The RFC is a little ambiguous here: “the subject field contains an empty
sequence” could mean that it's a non-empty sequence where one of the
sets contains an empty sequence. But, in context, I think it means “the
subject field is an empty sequence”.

Fixes #22249

Change-Id: Idfe1592411573f6e871b5fb997e7d545597a0937
Reviewed-on: https://go-review.googlesource.com/70852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>