campsite_cabin_incomplete_roof probably shouldn't have predecessor_mapgen set to "forest"

[PatrikLundell]

Describe the bug

Extract from a failed PR test, seemingly unrelated to the PR content:

(continued from above) ERROR : D:\a\Cataclysm-DDA\Cataclysm-DDA\src\mapgen.cpp:3169 [apply] In mapgen campsite_cabin_incomplete_roof on campsite_cabin_incomplete_roof_east, setting terrain to t_open_air (from t_region_groundcover) at (6,21,1) when furniture was f_region_forest. Resolve this either by removing the terrain from this mapgen, adding suitable removal commands to the mapgen, or by adding an appropriate clearing flag to the innermost layered mapgen. Consult the "mapgen flags" section in MAPGEN.md for options.
14:33:51.353 ERROR : D:\a\Cataclysm-DDA\Cataclysm-DDA\src\map.cpp:2333 [ter_set] Setting terrain t_open_air at (6,21,1) where furniture is f_region_forest. Terrain is_open_air
If this is intentional, set the ALLOW_ON_OPEN_AIR flag on the furniture
14:33:51.353 ERROR : D:\a\Cataclysm-DDA\Cataclysm-DDA\src\map.cpp:1830 [furn_set] Setting furniture f_wild_sarsaparilla at (6,21,1) where terrain is t_open_air (which is_open_air)

Attach save file

N/A. It's a random test result.

Steps to reproduce

Run tests until the cows come home...

Expected behavior

No test failure

Screenshots

No response

Versions and configuration

N/A (Whatever github runs).

Additional context

My analysis is that the predecessor_mapgen for campsite_cabin_incomplete_roof causes the area to initiate to forest terrain, despite being above ground, and then replace the terrain with the actual contents of the map, i.e. air and roof components.
Occasionally "furniture" is added (in this case a sarsaparilla flower) and you'd get this error when remains suspended when the ground is replaced by air.

The predecessor should probably default the terrain to air, but it might also work if there's some air predecessor_terrain available.