title: How to convert all columns in Pandas DataFrame to 'object' while ignoring NaN?
body: <p>I have a dataframe for which I want every column to be in string format. So I do this:</p>
  
  <pre><code> df = df.astype(str)
  </code></pre>
  
  <p>The problem is that in this way all the NaN entries are converted to string 'nan'. And <code>isnull</code> returns <code>false</code>. Is there a way to convert to string but keep empty entry as it is?</p>