title: Unexpected pandas.Series.replace() behavior
body: <p>Given this -</p>
  
  <p><code>import pandas as pd</code></p>
  
  <p><code>s = pd.Series(['', '1', '2', '', '4', '', '6'])</code></p>
  
  <p>Why does this -</p>
  
  <p><code>s.replace('', None).values</code></p>
  
  <p>Result in this -</p>
  
  <p><code>array(['', '1', '2', '2', '4', '4', '6'], dtype=object)</code></p>
  
  <p>When I would expect this -</p>
  
  <p><code>array([None, '1', '2', None, '4', None, '6'], dtype=object)</code></p>