title: How can I get an empty list instead of null?
body: <p>Suppose I have a couple of arrays:</p>
  
  <pre><code>$a = 1,2,3
  $b = 4,5,6
  </code></pre>
  
  <p>and I get the intersection (empty):</p>
  
  <pre><code>$c = $a |?{$b -contains $_}
  </code></pre>
  
  <p>I would expect <code>$c</code> to contain an empty list such that I could do:</p>
  
  <pre><code>$c.length
  </code></pre>
  
  <p>but I don't.  I get a <code>$null</code> (which issues an exception when queried for <code>.length</code>). How can I get an empty list back instead?</p>