[Bug] Ping exception when WS disconnected
**Describe the bug**
```
2023-05-16 15:16:56 - node           - ERROR    - Ping thread had an exception
Traceback (most recent call last):
  File "/vantage6/vantage6-node/vantage6/node/__init__.py", line 873, in __socket_ping_worker
    self.socketIO.emit('ping', namespace='/tasks')
  File "/usr/local/lib/python3.10/site-packages/socketio/client.py", line 393, in emit
    raise exceptions.BadNamespaceError(
socketio.exceptions.BadNamespaceError: /tasks is not a connected namespace.
```

**To Reproduce**
1. start node
2. restart the server (several times) 
diff --git a/vantage6-node/vantage6/node/__init__.py b/vantage6-node/vantage6/node/__init__.py
index 1cdaaa66..7463a3ec 100644
--- a/vantage6-node/vantage6/node/__init__.py
+++ b/vantage6-node/vantage6/node/__init__.py
@@ -870,7 +870,10 @@ class Node(object):
 
         while True:
             try:
-                self.socketIO.emit('ping', namespace='/tasks')
+                if self.socketIO.connected:
+                    self.socketIO.emit('ping', namespace='/tasks')
+                else:
+                    self.log.debug('SocketIO is not connected, skipping ping')
             except Exception:
                 self.log.exception('Ping thread had an exception')
             # Wait before sending next ping
['vantage6-node/vantage6/node/__init__.py']