title: Parse null from dictionary in Pandas
body: <p>I read a column from a db like this</p>
  
  <pre><code>row0:  {"colA":"valA", "colB":null}
  row1:  {"colA":"valC", "colB":null}
  </code></pre>
  
  <p>Python read each row as string, which I want to convert to dictionary. However I am unable to parse this because of the null value. How can I convert the null into 'null' or NaN so that I can treat it and later explode into individual columns. </p>
  
  <p>My final expected output is</p>
  
  <pre><code>colA  colB
  valA  NaN
  valC  NaN
  </code></pre>