[BUG] Error in indexer when ordering make on ibetExchange

[urushibata-k]

Describe the bug

The following error occurred in ibet-Wallet-API.
refs BoostryJP/ibet-Wallet-API#1099

[2022-01-17 02:47:30 +0000] [INDEXER-POSITION-MEMBERSHIP] [37] [INFO] syncing from=2461470, to=2461479                                                                                                                                   │
│   [2022-01-17 02:47:31 +0000] [INDEXER-POSITION-MEMBERSHIP] [37] [ERROR] Class 'builtins.NoneType' is not mapped                                                                                                                           │
│   Traceback (most recent call last):                                                                                                                                                                                                       │
│     File "batch/indexer_Position_Membership.py", line 331, in __sync_exchange                                                                                                                                                              │
│       self.sink.on_position(                                                                                                                                                                                                               │
│     File "batch/indexer_Position_Membership.py", line 67, in on_position                                                                                                                                                                   │
│       sink.on_position(*args, **kwargs)                                                                                                                                                                                                    │
│     File "batch/indexer_Position_Membership.py", line 115, in on_position                                                                                                                                                                  │
│       self.db.merge(position)                                                                                                                                                                                                              │
│     File "<string>", line 2, in merge                                                                                                                                                                                                      │
│     File "/home/apl/.pyenv/versions/3.8.7/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2969, in merge                                                                                                                      │
│       object_mapper(instance)  # verify mapped                                                                                                                                                                                             │
│     File "/home/apl/.pyenv/versions/3.8.7/lib/python3.8/site-packages/sqlalchemy/orm/base.py", line 296, in object_mapper                                                                                                                  │
│       return object_state(instance).mapper                                                                                                                                                                                                 │
│     File "/home/apl/.pyenv/versions/3.8.7/lib/python3.8/site-packages/sqlalchemy/orm/base.py", line 318, in object_state                                                                                                                   │
│       raise exc.UnmappedInstanceError(instance)                                                                                                                                                                                            │
│   sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped

Expected behavior

Being skiped merge when registering with 0 holdings.