innodb_redo_log_encrypt must not be settable when server is not started with keyring

While testing PS-4556, it is noticed that if server is started without keyring support, we are still able to execute below queries:

mysql> set global innodb_redo_log_encrypt=master_key;
Query OK, 0 rows affected (0.00 sec)

mysql> set global innodb_redo_log_encrypt=keyring_key;
Query OK, 0 rows affected (0.00 sec)

Ideally, it should return an error similar to:
ERROR 3185 (HY000): Can't find master key from keyring, please check in the server log if a keyring plugin is loaded and initialized successfully.