fixes scikit-learn-contrib#17

Author: anirudt

metric_learn/lmnn.py

@@ -206,8 +206,8 @@ def _find_impostors(self, furthest_neighbors):
         tmp = np.ravel_multi_index((i,j), shape)
         i,j = np.unravel_index(np.unique(tmp), shape)
       impostors.append(np.vstack((in_inds[j], out_inds[i])))
-    if not impostors:
-        """No imposters detected"""
+    if impostors.size == 0:
+        """No impostors detected"""
         return []
     return np.hstack(impostors)