title: RTextTools empty document
body: <p>After creating a matrix using the <code>create_matrix()</code> function in the package '<code>RTextTools</code>', I receive the following error:</p>
  
  <pre><code>train.matrix = create_matrix(cbind(train.corpus,train.pp.title), language="english",
  weighting=weightTfIdf)
  </code></pre>
  
  <p>Warning message:</p>
  
  <pre><code>In weighting(x) :
    empty document(s):
  </code></pre>
  
  <p>I believe this may be due the <code>weightTfIdf</code> weighting requiring that all rows that are filled in the corpus. </p>
  
  <p>What can I do to avoid this warning message?</p>