Skip pending nodes that have no handler

The guard evaluated the `next` builtin and discarded it, which does
nothing, so a pending node with no handler fell through to
None.NodeHandler(...).  The AttributeError was caught by the enclosing
except and logged as "Unexpected error during discovery", turning a node
that should have been quietly skipped into a spurious error in the log.

confluent_server/confluent/discovery/core.py

@@ -714,7 +714,7 @@ async def _recheck_nodes_backend(nodeattribs, configmanager):
         info = pending_nodes[nodename]
         try:
             if info['handler'] is None:
-                next
+                continue
             handler = info['handler'].NodeHandler(info, configmanager)
             tasks.spawn(eval_node(configmanager, handler, info, nodename))
         except Exception: