fix: always return empty array for selection on unknown topology

Topology types of `Unknown` should always return an empty set of
servers for server selection using a read preference

NODE-2549

Author: mbroadst

lib/core/sdam/server_selection.js

@@ -186,6 +186,10 @@ function readPreferenceServerSelector(readPreference) {
       );
     }
 
+    if (topologyDescription.type === TopologyType.Unknown) {
+      return [];
+    }
+
     if (
       topologyDescription.type === TopologyType.Single ||
       topologyDescription.type === TopologyType.Sharded