@Test
  void safeRebuffering_noContent() {
    server.enqueue(new MockResponse().setResponseCode(204));

    TestInterface api =
        newBuilder()
            .logger(
                new Logger() {
                  @Override
                  protected void log(String configKey, String format, Object... args) {}
                })
            .logLevel(Logger.Level.FULL) // rebuffers the body
            .target(TestInterface.class, "http://localhost:" + server.getPort());

    api.post("foo");
  }
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