Use empty string in Sse event when there is no data

Using an empty string instead of null
is what the classic rest client does,
so let's align with it

Closes: quarkusio#37033

independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/SseParser.java

@@ -158,7 +158,7 @@ private void dispatchEvent() {
  event.setComment(commentBuffer.length() == 0 ? null : commentBuffer.toString());
  // SSE spec says empty string is the default, but JAX-RS says null if not specified
  event.setId(lastEventId);
- event.setData(dataBuffer.length() == 0 ? null : dataBuffer.toString());
+ event.setData(dataBuffer.length() == 0 ? "" : dataBuffer.toString());
  // SSE spec says "message" is the default, but JAX-RS says null if not specified
  event.setName(eventType);
  event.setReconnectDelay(eventReconnectTime);