title: Replacing NaNs in a dataframe with a string value
body: <p>I want to replace the missing value in one column of my df with "missing value".
  I tried </p>
  
  <pre><code>result['emp_title'].fillna('missing')
  </code></pre>
  
  <p>or</p>
  
  <pre><code>result['emp_title'] = result['emp_title'].replace({ np.nan:'missing'})
  </code></pre>
  
  <p>the second one works, since when i count missing value after this code:</p>
  
  <pre><code>result['emp_title'].isnull().sum()
  </code></pre>
  
  <p>it gave me 0.
  However, the first one does not work as I expected, which did not give me a 0, instead of the previous count for missing value.
  Why the first one does not work? Thank you!</p>