Fix question mark tiles appearing under housing items

[brightrim]

Issue report: https://discord.com/channels/401855954272124940/1209503117784256523/1224857331989483541

Since invisible tiles can not be walked on without an invisible item, it should make no difference if the empty tiles are in fact empty or just invisible. So in order to avoid the question marks that happened in the issue reported above:

• Make a script that replaces any empty tiles on housing properties with invisible tiles
• Replace any instances of empty tiles (such as deleting tiles by replacing them with empty ones) with invisible tiles instead
• Ensure that this does not hide the upper floor by default somehow when on the lower floor, as a concern was raised about this. If it does, find alternative method.