title: Pandas apply to all values except missing
body: <p>Say I had a series such as:</p>
  
  <blockquote>
    <p>[1,2, NAN, 4]</p>
  </blockquote>
  
  <p>If I did:</p>
  
  <pre><code>series.apply(lambda a: a+2)
  </code></pre>
  
  <p>I would get an error due to the NAN. Is there an elegant way to preserve the NANs while applying my change?</p>