Remove uses of TransformComponent.ChildEntities (#22442)

* Make output of ChildEnumerator non-nullable

* Remove uses of ChildEntities

* poke tests

---------

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/Maps/ResaveCommand.cs

@@ -46,10 +46,9 @@ public sealed class ResaveCommand : LocalizedCommands
             {
                 loader.SaveMap(mapId, fn.ToString());
             }
-            else
+            else if (mapXform.ChildEnumerator.MoveNext(out var child))
             {
-
-                loader.Save(mapXform.ChildEntities.First(), fn.ToString());
+                loader.Save(child, fn.ToString());
             }
 
             _mapManager.DeleteMap(mapId);