title: Finding null values in a json record in a dataframe
body: <p>Let say we have a dataframe with the following format:</p>
  <pre><code>col1
  {'overall_mean': '93.07', 'overall_median': '91.05'}
  {}
  {'overall_mean': '91.02', 'overall_median': '95.03'}
  </code></pre>
  <p>The original data is in json format. I want to extract some elements from each row and here is what I tried:</p>
  <pre><code> df['col1'].apply(lambda col: col['overall_mean'])
  </code></pre>
  <p>If we have that element in the dictionary, everything is fine but if we don't have, it complains and it make sense. Then I changed to this one:</p>
  <pre><code> df['col1'].apply(lambda col: None if len(col['overall_mean']) == 1 else col['overall_mean'])
  </code></pre>
  <p>and the reason to choose this conditions <code>len(col['overall_mean']) == 1</code> was because that is the only condition that returns <code>True</code>. However it still cannot find those null value.</p>