handle server certificate is none

asyncua/client/client.py

@@ -468,6 +468,8 @@ class Client:
             data = self.security_policy.host_certificate + nonce
         self.security_policy.asymmetric_cryptography.verify(data, response.ServerSignature.Signature)
         self._server_nonce = response.ServerNonce
+        server_certificate = None
+        if response.ServerCertificate is not None:
             # If a server has certificate chain, the certificates are chained
             # this generates a error in our crypto part, so we strip everything after
             # the server cert. To do this we read byte 2:4 and get the length - 4