title: How to stop processing if observable is empty or null
body: <p>I am trying rxJava. I have a chain of operators which process the <code>Observable</code> returned from a service. </p>
  
  <p>I want to do nothing when the returned <code>Observable</code> is empty (<code>Observable.empty()</code>). </p>
  
  <p>How can I achieve that ?</p>