fix: sse event do not skip data field when empty

[co42]

Motivation

Since https://github.com/tokio-rs/axum/releases/tag/axum-v0.8.5 SSE events with empty data are sent without a data field.

Event::default().event("deleted").data("")

Before :

event: deleted
data: 

After

event: deleted

According to https://html.spec.whatwg.org/multipage/server-sent-events.html (I did not found an RFC)

If the data buffer is an empty string, set the data buffer and the event type buffer to the empty string and return.

Which means these events are ignored (if I understand it correctly).

Before this change the SSE client I use was receiving the event, it does not anymore.

More context : jpopesculian/eventsource-stream#6

Solution

Write empty data field if set to an empty string as before.

[jplatte]

Seems likely to be related to #3154, any thoughts @nanoqsh / @mladedav?

[co42]

Can it be merged ?