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

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 org.springframework.web.testfixture.servlet.MockHttpServletResponse;
 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 @@ class SseServerResponseTests {
 		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 {