Response body not parsed for 202 -- doesn't match HTTP spec or Graph API behavior

When the response code of an API call is 202 Accepted, then CoreHttpProvider calls handleEmptyResponse(), which ignores the response body.

This causes the copyNotebook operation in the microsoft-graph jar to return an OnenoteOperation object with all null properties, and so the id can't be used to query the operation's status.

The HTTP spec for 202 says "The representation sent with this response ought to describe the request's current status and point to (or embed) a status monitor that can provide the user with an estimate of when the request will be fulfilled." so I believe the API itself is behaving correctly/within the spec and that it is this project that is incorrectly assuming that a 202 response will not have any body content.

Expected behavior

graphClient.sites(site.id).onenote().notebooks(notebook.id).copyNotebook(params).buildRequest().post(); returns a OnenoteOperation with fields populated from the response body.

Actual behavior

graphClient.sites(site.id).onenote().notebooks(notebook.id).copyNotebook(params).buildRequest().post(); returns a OnenoteOperation with all fields set to null.

Steps to reproduce the behavior

SiteRequestBuilder siteRequestBuilder = graphClient.sites(site.id);
OnenoteOperation operation = siteRequestBuilder.onenote().notebooks(notebook.id).copyNotebook(params).buildRequest().post();
// operation.id is null here, so the following line fails:
operation = siteRequestBuilder .onenote().operations(operation.id).buildRequest().get();

Start in src/main/java/com/microsoft/graph/http/CoreHttpProvider.java at the handleEmptyResponse() call linked in the issue, then reproduce the copyNotebook request shown in the Java example. Done means a 202 Accepted response populates the returned OnenoteOperation fields, including its id, so the operation status can be queried.