Set encryption algo to None instead of empty string as default

opcua/client/client.py

@@ -446,7 +446,7 @@ class Client(object):
             if self._password:
                 self.logger.warning("Sending plain-text password")
                 params.UserIdentityToken.Password = password
-            params.UserIdentityToken.EncryptionAlgorithm = ''
+            params.UserIdentityToken.EncryptionAlgorithm = None
         elif self._password:
             data, uri = self._encrypt_password(password, policy_uri)
             params.UserIdentityToken.Password = data