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

[rboyer]

When using x509.CreateCertificate empty Subject fields correctly set the
SAN extension as critical. This changeset ensures that
x509.CreateCertificateRequest does the same thing.

This was previously fixed in x509.CreateCertificate with:
https://go-review.googlesource.com/70852

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