title: Can a Retry be configured to retry on null?
body: <p>I am just starting out with resilience4j and was wondering if it is possible to setup a retry to retry on null results. Here is an example:</p>
  
  <pre><code>Retry retry = Retry.ofDefaults("retry");
  Retry.decorateSupplier(retry, () -&gt; (String) null).get();
  assertThat(retry.getMetrics().getNumberOfSuccessfulCallsWithoutRetryAttempt()).isEqualTo(0);
  </code></pre>
  
  <p>This fails because returning null is ok. Is it possible to configure retry on null.</p>