fix: don't check non-unified topologies for session support check

Author: mbroadst

lib/core/cursor.js

@@ -586,7 +586,7 @@ Cursor.prototype._initializeCursor = function(callback) {
   const cursor = this;
 
   // NOTE: this goes away once cursors use `executeOperation`
-  if (cursor.topology.shouldCheckForSessionSupport()) {
+  if (cursor.topology.description != null && cursor.topology.shouldCheckForSessionSupport()) {
     cursor.topology.selectServer(ReadPreference.primaryPreferred, err => {
       if (err) {
         callback(err);