title: Pandas: AttributeError: 'str' object has no attribute 'isnull'
body: <p>I'm creating a new column named <strong>lead_actor_actress_known</strong> whose values is boolean based on whether there value in 2nd column <strong>lead_actor_actress</strong> has value or not. If there is a value(Name of actors) populate 1st column using <strong>True</strong> if there is no value, return <strong>False</strong></p>
  <blockquote>
  <p>AttributeError: 'str' object has no attribute 'isnull'</p>
  </blockquote>
  <p>My code is throwing an error above.</p>
  <pre><code>df['lead_actor_actress_known'] = df['lead_actor_actress'].apply(lambda x: True if x.isnull() else False)
  </code></pre>
  <p>What i'm i missing?</p>