title: Replace missing values that result from a pandas merge
body: <p>df1 </p>
  
  <pre><code>|Invoice #  |Date        |Amount       
  |12         |12/15/2015  |$10 
  |13         |12/16/2015  |$11 
  |14         |12/17/2015  |$12 
  </code></pre>
  
  <p>df2 </p>
  
  <pre><code>|Invoice #  |Date        |Amount
  |12         |1/16/2016   |$10 
  |14         |1/17/2016   |$12 
  </code></pre>
  
  <p>Merged = df1.merge(df2,how = left, on = Invoice #)</p>
  
  <pre><code>|Invoice #  |Date         |Amount
  |12         |12/15/2015   |$10
  |NaN        |NaN          |NaN
  |14         |1/17/2016    |$12
  </code></pre>
  
  <p>What I would like to do is take Invoice 13 that returned a NaN value in the merge and place it in a list.  Any ideas?</p>