Heart-beat log message potentially without session id

Issue: SPR-15937

Author: jhoeller

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompHeaderAccessor.java

@@ -429,7 +429,8 @@ else if (StompCommand.DISCONNECT.equals(getCommand())) {
 	@Override
 	public String getDetailedLogMessage(@Nullable Object payload) {
 		if (isHeartbeat()) {
-			return "heart-beat in session " + getSessionId();
+			String sessionId = getSessionId();
+			return "heart-beat" + (sessionId != null ? " in session " + sessionId : "");
 		}
 		StompCommand command = getCommand();
 		if (command == null) {