ssh: reject certificates with an empty principal

[XZ-Legendary-56]

parseCert appended every ValidPrincipals entry from the wire format
without validating it. A certificate with a single empty principal
parsed successfully and passed len(cert.ValidPrincipals) > 0 checks,
so it appeared principal-restricted when it was not: CheckCert("")
matched the empty entry and returned success, and code inspecting
ValidPrincipals directly was misled the same way.

An empty principal has no meaningful interpretation, since an empty
principals list already means the certificate is valid for all users
and hosts. Reject it during parsing so every consumer is covered.

Fixes golang/go#81539

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