title: Pandas: remove empty strings from dataframe
body: <p>I have dataframe</p>
  
  <pre><code>id    m1    m2    m3
  111   20    0     12
  222   0     0     0
  333   3     1     18
  </code></pre>
  
  <p>I need to get only</p>
  
  <pre><code>id    m1    m2    m3
  111   20    0     12
  333   3     1     18
  </code></pre>
  
  <p>I use</p>
  
  <pre><code>df.drop(axis=0, how='all')
  </code></pre>
  
  <p>But it returns me full dataframe.
  How can I fix that?</p>