edge case where union is 0 for jaccard

logmetric.py

@@ -104,6 +104,9 @@ class LogMetric(evaluate.Metric):
     def get_jaccard_similarity(self, set1, set2):
         intersection = set1.intersection(set2)
         union = set1.union(set2)
+        if (len(union) == 0):
+            return 1.0
+
         return len(intersection) / len(union)
     
     # A score depending on the difference in length of two sentences