asn1.NullRawValue was used according to RFC 3279 2.2.1. Without this tag,
the output didn't match openssl.

Fixes #19972

Change-Id: Ia52ddb810888837f913dbd65c4e1328f6c8084bb
Reviewed-on: https://go-review.googlesource.com/40730Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>