How to handle empty response in Spring RestTemplate

Use String instead of AuthenticationResponse. If you get an empty response, the String value will be empty.

ResponseEntity<String> authenticateResponse = restTemplate.getForEntity(authenticateUrl, String.class);

UPDATE:
Check this link. I think this will fix your issue.

ResponseEntity<Void> response = restTemplate.getForEntity(authenticateUrl,Void.class);
Void body = response.getBody();