[TLS] Empty CertificateRequest.supported_signature_algorithms in TLS 1.2

[Martin Thomson <[e-mail]>]

In attempting to fix a bug related to this, a question came up about
what the semantics of an empty value is here.  Some implementations
seem to infer that empty means {*,SHA1}, which effectively assumes
that an empty value is equivalent to an absent signature_algorithms
extension (Section 7.4.1.4.1)

The text on CertificateRequest is less clear about what to do.  That's
understandable because it doesn't have to deal with the value being
absent because it's not optional.  All we have to go on is this from
Section 7.4.8:

   The hash and signature algorithms used in the signature MUST be
   one of those present in the supported_signature_algorithms field
   of the CertificateRequest message.

We think that treating an empty supported_signature_algorithms field
as an error is the best response and plan to implement that change.
We'll send a fatal alert if we receive one.

This is consistent with our handling of the signature_algorithms
extension, where we treat an empty list as a failure.