title: Pandas Use Value if Not Null, Else Use Value From Next Column
body: <p>Given the following dataframe:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'COL1': ['A', np.nan,'A'], 
                     'COL2' : [np.nan,'A','A']})
  df
      COL1    COL2
  0    A      NaN
  1    NaN    A
  2    A      A
  </code></pre>
  
  <p>I would like to create a column ('COL3') that uses the value from COL1 per row unless that value is null (or NaN). If the value is null (or NaN), I'd like for it to use the value from COL2.</p>
  
  <p>The desired result is:</p>
  
  <pre><code>    COL1    COL2   COL3
  0    A      NaN    A
  1    NaN    A      A
  2    A      A      A
  </code></pre>
  
  <p>Thanks in advance!</p>