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.
Creating a throwaway map just to query the tileshape won't work in TS, as the no-args constructor doesn't set this correctly.  This is a mod-level setting, so you can query it directly from the manifest.