title: case when null evaluates to false
body: <p>Today I was surprised by this <code>case</code> behaviour:</p>
  
  <pre><code>select case when null then true else false end;
   case 
  ------
   f
  </code></pre>
  
  <p>I would expect it to return <code>null</code> since a <code>null</code> casted to boolean yelds a <code>null</code> not a <code>false</code>:</p>
  
  <pre><code>select null::boolean is null;
   ?column? 
  ----------
   t
  </code></pre>
  
  <p>Any comments on the rationale of this behaviour? What Am I missing?</p>