Allow sending SSE events without data

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

Author: bclozel

spring-webmvc/src/main/java/org/springframework/web/servlet/function/ServerResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -570,6 +570,15 @@ interface SseBuilder {
 		 */
 		void send(Object object) throws IOException;
 
+		/**
+		 * Sends the buffered content as a server-sent event, without data.
+		 * Only the {@link #event(String) events} and {@link #comment(String) comments}
+		 * will be sent.
+		 * @throws IOException in case of I/O errors
+		 * @since 6.1.4
+		 */
+		void send() throws IOException;
+
 		/**
 		 * Add an SSE "id" line.
 		 * @param id the event identifier

spring-webmvc/src/main/java/org/springframework/web/servlet/function/SseServerResponse.java

@@ -135,6 +135,23 @@ public void send(Object object) throws IOException {
 			data(object);
 		}
 
+		@Override
+		public void send() throws IOException {
+			this.builder.append('\n');
+			try {
+				OutputStream body = this.outputMessage.getBody();
+				body.write(builderBytes());
+				body.flush();
+			}
+			catch (IOException ex) {
+				this.sendFailed = true;
+				throw ex;
+			}
+			finally {
+				this.builder.setLength(0);
+			}
+		}
+
 		@Override
 		public SseBuilder id(String id) {
 			Assert.hasLength(id, "Id must not be empty");
@@ -186,20 +203,7 @@ private void writeString(String string) throws IOException {
 			for (String line : lines) {
 				field("data", line);
 			}
-			this.builder.append('\n');
-
-			try {
-				OutputStream body = this.outputMessage.getBody();
-				body.write(builderBytes());
-				body.flush();
-			}
-			catch (IOException ex) {
-				this.sendFailed = true;
-				throw ex;
-			}
-			finally {
-				this.builder.setLength(0);
-			}
+			this.send();
 		}
 
 		@SuppressWarnings("unchecked")

spring-webmvc/src/test/java/org/springframework/web/servlet/function/SseServerResponseTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,8 +32,10 @@
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
+ * Tests for {@link ServerResponse.SseBuilder}.
  * @author Arjen Poutsma
  * @author Sebastien Deleuze
+ * @author Brian Clozel
  */
 class SseServerResponseTests {
 
@@ -151,6 +153,26 @@ void builder() throws Exception {
 		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
 	}
 
+	@Test
+	void sendWithoutData() throws Exception {
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.event("custom").send();
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException(ex);
+			}
+		});
+
+		ServerResponse.Context context = Collections::emptyList;
+
+		ModelAndView mav = response.writeTo(this.mockRequest, this.mockResponse, context);
+		assertThat(mav).isNull();
+
+		String expected = "event:custom\n\n";
+		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
+	}
+
 
 	private static final class Person {