title: Column value replace
body: <p>I have a pandas dataframe whose col_3 is <code>df['col_1].map(str) +','+df['col_2].map(str)</code></p>
  
  <pre><code>col_1         col_2       col_3
  abccd                    ['abccd','nan']
  mmdf          Hgfg       ['mmdf','Hgfg']
  abccd         ddsx       ['abccd','ddsx']
  hhdy                     ['hhdy','nan']
  </code></pre>
  
  <p>Clearly the 'nan' is being created as Col_2 is blank. 
  My question is: How can I replace the 'nan' with Col_1 values i.e. whenever Col_2 value is empty Col_3 would be <code>df['col_1].map(str) +','+df['col_1].map(str)</code>
  Please help</p>