title: Preserve Nulls in Pandas dataframe
body: <p>I have a df</p>
  <pre><code>a=pd.DataFrame({'A':[1,2,3,4], 'B':[1.0,NaN,3.0,NaN]})
  </code></pre>
  <p>I want to convert column B to string preserving the nulls. Please suggest a method on how to achieve it.</p>