title: Pandas, 'nan' fields after changing column datatypes
body: <p>I am changing the datatype of a determined columns in pandas and I would like to keep the nan values as np.nan</p>
  <p>After executing the following command:</p>
  <pre><code>df = df.astype({&quot;raw_salary_from&quot;: str, &quot;raw_salary_to&quot;: str})
  </code></pre>
  <p>What I get is that all the nan's transformed into strings 'nan'. How can I avoid that issue while transforming the datatype of the column?</p>
  <pre><code>['22000.0',
   '0.0',
   '40000.0',
   '9.5',
   '0.0',
   '0.0',
   '28.0',
   'nan',
   'nan',
   'nan',
   'nan',
   'nan']
  </code></pre>