title: Selecting rows where column is not null and equal to a (string) value
body: <p>So basically I want select all rows where Col A is equal to the string 'hey'.
  My problem is that Col A can contain null/nan's therefore I get a </p>
  
  <pre><code>TypeError: invalid type comparison. 
  </code></pre>
  
  <p>When executing:</p>
  
  <pre><code>df.loc[df['A'] == 'hey']
  </code></pre>
  
  <p>I then made another condition: </p>
  
  <pre><code>df.loc[df['A'].notnull() &amp; (df['A'] == 'hey')] 
  </code></pre>
  
  <p>Here i get the same error. </p>
  
  <p>I made a hack where I change all the null values in Col A to '' but thats not beautiful is there anyway nice to first choose all the rows where Col A isn't null and then from there all the ones who are equal to 'hey'? </p>