If a TLS connection results in the server choosing an anonymous cipher suite, then no server certificate will be returned by the server. This ought not to happen, since XMPP clients are expected only to propose non-anonymous cipher suites, but it could be that a client is coded to propose anonymous suites, or that a bug in the server means that it fails to return a server certificate. This change updates the ServerIdentityVerifier to make it resilient against these situations, treating this situation as equivalent to "certificate presented by server does not verify". Test-information: In my testing, I was deliberately using anonymous ciphers and getting Stroke crashes. After this patch, I don't get Stroke crashes any more (but the connection fails because the certificate verification fails). Change-Id: Ia7b9b8dad7a054ff266a78ef33a56157320654c8