x509: encode empty sequence as constructed

x509: encode empty sequence as constructed

According to the ASN.1 BER spec, we should be encoding
all sequences (including empty ones) as constructed:

	8.9.1 The encoding of a sequence value shall be constructed.
	8.10.1 The encoding of a sequence-of value shall be constructed.
	8.11.1 The encoding of a set value shall be constructed.
	8.12.1 The encoding of a set-of value shall be constructed.

However, we were only setting them as constructed when the
list was non-empty.

This changes it, and makes letsencrypt happy with the CSRs that
we generate.