Fix null request bodies for PUT requests

[ferid333]

Summary

Fixes null request bodies for PUT requests by sending an empty request body instead of passing null to OkHttp.

Problem

While reading through the request-building code, I noticed that POST requests with a null body were already converted to an empty byte array before building the OkHttp request. However, PUT requests were not handled the
same way.

When a PUT endpoint is called with a null request object, the client can call:

requestBuilder.method("PUT", null)                                                                                                                                                                                                

Expected Behavior

The client should build a valid PUT request with a zero-length request body, matching the existing behavior for null-body POST requests.

Changes

• Converts null bodies for POST and PUT requests into an empty byte array.
• Adds test coverage for a null-body PUT request.