Switch to empty SSE comment events for keep-alive

Prior to this commit, the SSE handler was using a "comment" event with a
single space character for keep-alive purposes because Spring Framework
did not support empty comments.

This has been fixed in Framework, we can now use empty comments.

Closes gh-1431

Author: bclozel

spring-graphql/src/main/java/org/springframework/graphql/server/webmvc/GraphQlSseHandler.java

@@ -165,9 +165,7 @@ private void sendNext(Map<String, Object> value) {
 
 		private void sendHeartbeat() {
 			try {
-				// Currently, comment cannot be empty:
-				// https://github.com/spring-projects/spring-framework/issues/34608
-				this.sseBuilder.comment(" ");
+				this.sseBuilder.comment("");
 				this.sseBuilder.send();
 			}
 			catch (IOException exception) {

spring-graphql/src/test/java/org/springframework/graphql/server/webmvc/GraphQlSseHandlerTests.java

@@ -146,13 +146,13 @@ void shouldSendKeepAlivePings() throws Exception {
 
 		assertThat(response.getContentAsString())
 				.startsWith("""
-					:\s
+					:
 
-					:\s
+					:
 
 					""")
 				.endsWith("""
-					:\s
+					:
 
 					event:complete
 					data: