storage: fix TypeError while stopping backup

This fixes the following error:

PACKET #0x0012 AnswerTruncate                 > M1 (127.0.0.3:1)
ERROR  Pre-mortem data:
ERROR  Traceback (most recent call last):
ERROR    File "neo/storage/app.py", line 166, in run
ERROR      self._run()
ERROR    File "neo/storage/app.py", line 207, in _run
ERROR      self.doOperation()
ERROR    File "neo/storage/app.py", line 308, in doOperation
ERROR      task_queue[-1].next()
ERROR    File "neo/storage/handlers/storage.py", line 240, in push
ERROR      conn.notify(Packets.AddObject(oid, serial, *object[2:]))
ERROR  TypeError: 'bool' object is unsubscriptable

neo/storage/handlers/storage.py

@@ -233,9 +233,9 @@ class StorageOperationHandler(EventHandler):
                                 del object_dict[serial]
                             continue
                     object = dm.getObject(oid, serial)
-                    if object is None:
+                    if not object:
                         conn.answer(Errors.ReplicationError(
-                            "partition %u dropped" % partition))
+                            "partition %u dropped or truncated" % partition))
                         return
                     conn.notify(Packets.AddObject(oid, serial, *object[2:]))
                     yield