fix RegisterWithDiscoveryServer method not using a certificate for the secure channel (#3005)

Author: romanett

Libraries/Opc.Ua.Server/Server/StandardServer.cs

@@ -2394,7 +2394,7 @@ public bool RegisterWithDiscoveryServer()
                                 requestHeader.Timestamp = DateTime.UtcNow;
 
                                 // create the client.
-                                var instanceCertificate = InstanceCertificateTypesProvider.GetInstanceCertificate(null);
+                                var instanceCertificate = InstanceCertificateTypesProvider.GetInstanceCertificate(endpoint.Description?.SecurityPolicyUri ?? SecurityPolicies.None);
                                 client = RegistrationClient.Create(
                                     configuration,
                                     endpoint.Description,