Update create_pascal_tf_record.py

when creat a tf record using purely negative images which have no 'object' in their xmls ,the script gets a key error : for obj in data['object']:  Key Error : 'object'
In order to fix this , add a 'if ' judegement before it .

research/object_detection/dataset_tools/create_pascal_tf_record.py

@@ -104,6 +104,8 @@ def dict_to_tf_example(data,
   truncated = []
   poses = []
   difficult_obj = []
+    
+  if data.has_key('object'):
     for obj in data['object']:
       difficult = bool(int(obj['difficult']))
       if ignore_difficult_instances and difficult: