title: Pandas - check if ALL values are NaN in Series
body: <p>I have a data series which looks like this:</p>
  
  <pre><code>print mys
  
  id_L1
  2       NaN
  3       NaN
  4       NaN
  5       NaN
  6       NaN
  7       NaN
  8       NaN
  </code></pre>
  
  <p>I would like to check is all the values are NaN. </p>
  
  <p>My attempt:</p>
  
  <pre><code>pd.isnull(mys).all()
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>True
  </code></pre>
  
  <p>Is this the correct way to do it? </p>