title: Replace empty string with with np.nan but got 'NaN'
body: <p>I tried to replace empty string with <code>np.nan</code></p>
  
  <p>But I got <code>None</code> in cells , what wrong with that ?</p>
  
  <p>Thanks</p>
  
  <pre><code>df.replace('', np.nan)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/bPz8z.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bPz8z.png" alt="enter image description here"></a></p>