title: Can't remove empty lines with sed regex
body: <p>I have a file like this:</p>
  
  <pre><code>2733617     3.00    3   3
  
  2733617 E1b1    8.00    8   16
  2733617 E1b1b1b 2.00    2   4
  
  2733617 I1  294.00  296 590
  2733617 I2  1.00    1   2
  
  2733617 I2a1    2.00    2   4
  </code></pre>
  
  <p><code>sed '/^$/d'</code> does not work for me. Outfile looks just like infile. It should remove empty lines.</p>