Improve setting of WebSocket error status

See gh-28836

Author: FlorianKirmaier

spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/AbstractListenerWebSocketSession.java

@@ -33,6 +33,7 @@
 import org.springframework.http.server.reactive.AbstractListenerWriteProcessor;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 import org.springframework.web.reactive.socket.CloseStatus;
 import org.springframework.web.reactive.socket.HandshakeInfo;
 import org.springframework.web.reactive.socket.WebSocketHandler;
@@ -247,7 +248,12 @@ public void onError(Throwable ex) {
 		if (this.handlerCompletionMono != null) {
 			this.handlerCompletionMono.onError(ex);
 		}
-		close(CloseStatus.SERVER_ERROR.withReason(ex.getMessage()));
+		if(!StringUtils.hasText(ex.getMessage())) {
+			close(CloseStatus.SERVER_ERROR);
+		}
+		else {
+			close(CloseStatus.SERVER_ERROR.withReason(ex.getMessage()));
+		}
 	}
 
 	@Override