"transient" set to None explicitly breaks execute

I am trying to get to the bottom of [this issue with deno kernel](https://github.com/rgbkrk/denotebooks/issues/8) and so I managed to track it down to the [following](https://github.com/jupyter/nbclient/blob/main/nbclient/client.py#L1103):

```python
display_id = content.get("transient", {}).get("display_id", None)
```

throws when receiving message with the following content (note explicit `None` which causes the issue):

`{'execution_count': 7, 'data': {'text/plain': '\x1b[32m"2.29.4"\x1b[39m'}, 'metadata': {}, 'transient': None}`

Should this be addressed in nbclient or is this an issue with the kernel?