In my opinion, others have already answered it via answers as well as comments. But since the question is still open, let me try to answer with some more details.
Your assumption that you should be able to get a JSON result as part of PUT API response is not always correct, it depends on the implementation of the API.
Here are different ways a PUT API can be implemented:
PUT API giving the object in response:
[HttpPut]
public HttpResponseMessage Put(int id, Product product)
{
//save the Product object.
return Request.CreateResponse(HttpStatusCode.OK, product);
}
In this implementation, the API gives the object in the response, this is what you are expecting.
PUT API giving empty response:
[HttpPut]
public void Put(int id, Product product)
{
//save the Product object.
}
In this implementation, API returns nothing in response.
Based on your explanation, the API you are calling is following the second way. You can verify this if you have access to the API code.
So, if your only problem is to know whether the API is not working or not, execute your code to do a PUT and then do a GET on the same object to verify if the update has been successful.
Hope this helps!
