Fixed a NullReferenceException in auto map download.

Spotted while testing https://github.com/OpenRA/OpenRA/pull/5839. Introduced by https://github.com/OpenRA/OpenRA/commit/a30c8b53a7ed8735ec55688ec0769774af76fb4a#diff-7. This fix may create wrong spawn point preview positions for not yet downloaded isometric maps.
This is also why the `??` operator exist.