title: pandas check if column is null with query function
body: <p>I have pandas dataframe that I want to execute on it query function with isnull() or not isnull() condition like that:</p>
  
  <pre><code>In [67]: df_data = pd.DataFrame({'a':[1,20,None,40,50]})
  In [68]: df_data
  Out[68]:       a
           0   1.0
           1  20.0
           2   NaN
           3  40.0
           4  50.0
  </code></pre>
  
  <p>if I use this command:</p>
  
  <pre><code>df_data.query('a isnull', engine='python')
  </code></pre>
  
  <p>or this command:</p>
  
  <pre><code>df_data.query('a isnull()', engine='python')
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>In [75]: df_data.query('a isnull', engine='python')  
  File "&lt;unknown&gt;", line 1    a isnull           
  SyntaxError: invalid syntax
  
  In [76]: df_data.query('a isnull()', engine='python')  
  File "&lt;unknown&gt;", line 1    a isnull ()           
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>What is the right way to do that?</p>
  
  <p>Thank you.</p>