Prior to this commit, the SseBuilder API in WebMvc.fn would only allow
to send Server-Sent Events with `data:` items in them. The spec doesnn't
disallow this and other specifications rely on such patterns to signal
the completion of a stream, like:
```
event:next
data:some data
event:complete
```
This commit adds a new `send()` method without any argument that sends
the buffered content (id, event comment and retry) without data.
Fixes: gh-32270
f9ae54d91e