title: how to make a null query selecting all the items
body: <p>I really like the <code>query</code> method. What is the <code>null</code> query that select all the items? I have tried <code>df.query("True")</code>, but it doesn't work.</p>
  
  <p>The only thing I have found to work is <code>df.query("index == 0 | index != 0")</code>.</p>
  
  <p>Why "True" is not working, it is a predicate.</p>