Fix error in Jaccard distance calculation for all zero points

Author: lmcinnes

hdbscan/dist_metrics.pyx

@@ -789,6 +789,8 @@ cdef class JaccardDistance(DistanceMetric):
             tf2 = x2[j] != 0
             nnz += (tf1 or tf2)
             n_eq += (tf1 and tf2)
+        if nnz == 0 and n_eq == 0:
+            return 0.0
         return (nnz - n_eq) * 1.0 / nnz