title: Jaccard Similarity Between Null Sets
body: <p>I want to compute the Jaccard similarity between two data sets based on the existence/nonexistence of a list of standard codes.
  For example (x,y,z are data sets):
  Data sets x and y don't have any standard codes (Null), therefore I set the list values as zeroes.</p>
  
  <pre><code> x = [0,0,0] 
   y = [0,0,0] 
   z = [0,1,0] 
  
  from sklearn.metrics import jaccard_similarity_score
  jaccard_similarity_score(x,y),jaccard_similarity_score(x, z)
  </code></pre>
  
  <p>Jaccard sim between x and z is 0.66 (2/3). Is there any similarity measure that deals with set intersection between two empty sets? In my case, I want to set the similarity between data set x and y as 0, not 1 (as computed using Jaccard). </p>