title: Removing null values in a data frame
body: <p>I'm really new to R so it would be great if there is an solution I can easily understand.
  I have a data set which contains two columns, a date and a price, and the price can be null in some cases.
  I tried to remove these values with <code>na.omit</code>, <code>complete.cases</code>, but it seems they are just for NA-values.</p>
  
  <p>The rows look like this</p>
  
  <pre><code>2017-05-31 12615.059570
  2017-06-01 12664.919922
  2017-06-02 12822.940430
  2017-06-05         null
  </code></pre>
  
  <p>So is there a way to remove null-values in a data frame?</p>