[Core] Queueing an empty response can hang the connection

## Hangs

```python
def handle_request(self, request: HttpParser) -> None:
        self.client.queue(okResponse())
```

## Works

```python
def handle_request(self, request: HttpParser) -> None:
        self.client.queue(okResponse(content=b'Hello World'))
```

This also happens for `HttpProxyPlugin` when trying to dispatch an empty response.  See #1039 and #1040 for background.