@Test
  void noResponseBodyForPost() throws Exception {
    server.enqueue(new MockResponse());

    TestInterface api =
        newBuilder().target(TestInterface.class, "http://localhost:" + server.getPort());

    api.noPostBody();
  }
This shows that is a no-op or otherwise doesn't cause an NPE when there's no content.
core/src/test/java/feign/client/AbstractClientTest.java