修复Session关闭时空指针问题

kernel-d-socket/socket-sdk-websocket/src/main/java/cn/stylefeng/roses/kernel/socket/websocket/session/SessionCenter.java

@@ -103,7 +103,9 @@ public class SessionCenter {
      **/
     public static void closed(String sessionId) {
         for (List<SocketSession<GettySocketOperator>> values : socketSessionMap.values()) {
+            if (ObjectUtil.isNotEmpty(values)) {
                 values.removeIf(item -> item.getSessionId().equals(sessionId));
             }
         }
     }
+}