title: Pandas series replace empty string values in a column with nan
body: <p>I have the following "TL" column in a pandas dataframe: </p>
  
  <pre class="lang-none prettyprint-override"><code>ID  TL     
  1   alfdjalk
  2   jafd;jsa
  3
  4
  5   ajf;dlka;fd
  6   ajdf;k
  </code></pre>
  
  <p>Right now the TL has dtype of object.  The blanks are empty strings.  </p>
  
  <p>I want to make the empty strings nulls instead.  So I tried the following code: </p>
  
  <pre><code>df_EVENT5_28['TL'] = df_EVENT5_28['TL'].apply(lambda x: x.str.strip()).replace('', np.nan)
  </code></pre>
  
  <p>But I'm getting the error below: AttributeError: 'str' object has no attribute 'str'</p>
  
  <p>Thoughts? </p>