Empty-segment Historicals not detected as Dart workers

Historical nodes with no segments are not recognized by BrokerServerView, so they cannot be used as Dart workers. This prevents query execution on fresh clusters or clusters with only realtime data. The fix should add these empty Historicals to the server view.

Start by locating BrokerServerView and tracing how Historical nodes are classified as Dart workers. Reproduce the case with an empty-segment Historical on a fresh or realtime-only cluster, then verify that it appears in the server view and can execute queries as a Dart worker.

Written by the indexing model from the issue text.