Some fixes to preferred_dir default handling

Author: vidartf

jupyter_server/serverapp.py

@@ -1835,6 +1835,9 @@ def init_configurables(self):
             parent=self,
             log=self.log,
         )
+        # Trigger a default/validation here explicitly while we still supporte
+        # deprecated trait on ServerApp (FIXME remove when deprecation finalized)
+        self.contents_manager.preferred_dir
         self.session_manager = self.session_manager_class(
             parent=self,
             log=self.log,