title: how to add junit testcase for null hashmap
body: <p>I have below code snippet of hashmap</p>
  <pre><code>if (defgmap==null){
  
  return Collections.emptylist();
  }
  </code></pre>
  <p>How can we add junit test case / using mockito object for the same null check .As I am new to this, any leads would be helpful.
  What about using assertnull() ?</p>