nixos/services.xserver: Fix legacy options for default wm without dm

We switched to unified default session option services.xserver.displayManager.defaultSession
and included fallback path for the legacy options. Unfortunately when only
services.xserver.windowManager.default is set and not services.xserver.desktopManager.default,
it got incorrectly converted to the new option.

This should fix that.

Closes: https://github.com/NixOS/nixpkgs/issues/76684