Schema generation of defaults is incompatible with Avro schema

### Checks

- [x] I added a descriptive title to this issue
- [x] I have searched (google, github) for similar issues and couldn't find anything

# Bug

- py2k version: `1.9.1`
- Python version: `Any`
- Operating System: `Any`

## Expected Result
When generating a Schema with a default, it should look like:
```py
{"name": "xxx", "type": ["null", "boolean"], "default": null}
```

## Actual Result

```py
{"name": "xxx", "type": "boolean", "default": null}
```