Allow empty comment in ServerResponse.SseBuilder

Closes gh-34608

Author: rstoyanchev

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -173,7 +173,6 @@ public SseBuilder retry(Duration duration) {
 
 		@Override
 		public SseBuilder comment(String comment) {
-			Assert.hasLength(comment, "Comment must not be empty");
 			String[] lines = comment.split("\n");
 			for (String line : lines) {
 				field("", line);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -173,6 +173,26 @@ void sendWithoutData() throws Exception {
 		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
 	}
 
+	@Test // gh-34608
+	void sendHeartbeat() throws Exception {
+		ServerResponse response = ServerResponse.sse(sse -> {
+			try {
+				sse.comment("").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 = ":\n\n";
+		assertThat(this.mockResponse.getContentAsString()).isEqualTo(expected);
+	}
+
 
 	private static final class Person {