crypto/tls: signature_algorithms in CertificateRequest can't be empty

crypto/tls: signature_algorithms in CertificateRequest can't be empty

Change-Id: I6a6a4656ab97e1f247df35b2589cd73461b4ac76
Reviewed-on: https://go-review.googlesource.com/c/go/+/675917
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>