Adding a streamed empty LevelInstance in a WP level will result in an incorrect mapcheck error

This is an edge case that can also be caused when a Level only contains actors were `SupportsExternalPackaging` is false.  This is likely the case for the "empty" level which surely contains a LevelScriptActor. 

• Create a new WP level
• Create an Empty Level
• Use the Empty Level as a LevelInstance. 
• Run MapCheck

MapCheck: Error: Level Instance /Game/NewMap.EmptyLevel is not using external actors, resave level to add compatibility /Game/EmptyLevel.EmptyLevel