dbapi2.Cursor.description property type annotation should be Optional[...]

The `dbapi2.Cursor._description` can be null, so the accessor should be an `Optional` type.

```python
>>> from comdb2 import dbapi2
>>> conn = dbapi2.connect("....")
>>> cursor = conn.cursor()
>>> type(cursor.description)
<class 'NoneType'>
```

The type annotation for the property is `tuple[tuple[str, object, None, None, None, None, None], ...]`