title: How to convert type to bool in pandas with `None` values in the series?
body: <p>Why does the value <code>None</code> convert to both <code>True</code> and <code>False</code> in this series?</p>
  <p>Env:</p>
  <ul>
  <li>Jupyter Notebok 6.0.3 in Jupyter Labs</li>
  <li>Python 3.7.6</li>
  </ul>
  <p>Imports:</p>
  <pre><code>from IPython.display import display
  import pandas as pd
  </code></pre>
  <p>Converts <code>None</code> to <code>True</code>:</p>
  <pre><code>df_test1 = pd.DataFrame({'test_column':[0,1,None]})
  df_test1['test_column'] = df_test1.test_column.astype(bool)
  display(df_test1)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/dWrp9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dWrp9.png" alt="enter image description here" /></a></p>
  <p>Converts <code>None</code> to <code>False</code>:</p>
  <pre><code>df_test2 = pd.DataFrame({'test_column':[0,1,None,'test']})
  df_test2['test_column'] = df_test2.test_column.astype(bool)
  display(df_test2)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/U3wac.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U3wac.png" alt="enter image description here" /></a></p>
  <p>Is this expected behavior?</p>