`UnboundLocalError` in `email.message.Message.get_payload`

# Bug report

### Bug description:

```python
    html_body = part.get_payload(decode=True).decode()
  File "/usr/local/lib/python3.8/email/message.py", line 282, in get_payload
    return quopri.decodestring(bpayload)
```


### CPython versions tested on:

CPython main branch

### Operating systems tested on:

Linux


### Linked PRs
* gh-136071