Merge pull request #4507 from guoshengCS/fix-infer-none

Fix infer when input is empty in v2/inference.py

python/paddle/v2/inference.py

@@ -96,6 +96,9 @@ class Inference(object):
             for i, item in enumerate(result):
                 retv[i].append(item)
 
+        if retv == None:
+            return []
+
         if flatten_result:
             retv = [numpy.concatenate(out) for out in retv]