unit test coverage missing `__arrow_array__` methods

```
db_dtypes/__init__.py             105      2     36      0    99%   100, 160
```

https://github.com/googleapis/python-db-dtypes-pandas/blob/bc9f34d9930f0a394c4e3fd52bc61f71c597aebf/db_dtypes/__init__.py#L100

https://github.com/googleapis/python-db-dtypes-pandas/blob/bc9f34d9930f0a394c4e3fd52bc61f71c597aebf/db_dtypes/__init__.py#L160