L2SS-465: Support get_node([]) to avoid a breaking edge case

tangostationcontrol/tangostationcontrol/clients/opcua_client.py

@@ -136,6 +136,9 @@ class OPCUAConnection(AsyncCommClient):
     async def get_node(self, path):
         """ Retrieve an OPC-UA node from either the cache, or the server. """
 
+        if not path:
+            return self.obj
+
         cache_key = ",".join(path)
 
         # lookup in cache