For records that undo object creation, None values are used at the backend
level whereas the protocol is not designed to serialize None for any field.

Therefore, a dance done in many places around packet serialization, using the
specific 0/ZERO_HASH/'' triplet to represent a deleted oid. For replication,
it was missing at the sender side, leading to the following crash:

  Traceback (most recent call last):
    File "neo/storage/app.py", line 147, in run
      self._run()
    File "neo/storage/app.py", line 178, in _run
      self.doOperation()
    File "neo/storage/app.py", line 257, in doOperation
      next(task_queue[-1]) or task_queue.rotate()
    File "neo/storage/handlers/storage.py", line 271, in push
      conn.send(Packets.AddObject(oid, *object), msg_id)
    File "neo/lib/protocol.py", line 234, in __init__
      self._fmt.encode(buf.write, args)
    File "neo/lib/protocol.py", line 345, in encode
      return self._trace(self._encode, writer, items)
    File "neo/lib/protocol.py", line 334, in _trace
      return method(*args)
    File "neo/lib/protocol.py", line 367, in _encode
      item.encode(writer, value)
    File "neo/lib/protocol.py", line 345, in encode
      return self._trace(self._encode, writer, items)
    File "neo/lib/protocol.py", line 342, in _trace
      raise ParseError(self, trace)
  ParseError: at add_object/checksum:
    File "neo/lib/protocol.py", line 553, in _encode
      assert len(checksum) == 20, (len(checksum), checksum)
  TypeError: object of type 'NoneType' has no len()