Adds type comparison in Bolt\Config::get()

When evaluating if a value is null in the config it should do a type comparison. I'm having a issue when a set `session_use_storage_handler: false` https://github.com/bolt/bolt/blob/master/app/config/config.yml.dist#L213 and in the https://github.com/bolt/bolt/blob/master/app/src/Bolt/Application.php#L53 is returning `NULL`