fix: deconstructing terrain into t_null creates roofs or open air automatically, don't spawn deconstruction results in midair

[chaosvolt]

Purpose of change (The Why)

This is an alternative put together to fix behavior wherein deconstructing stairs creates patches of nothingness. Now with a fix for it dumping the wood down into the hole.

Closes #6141

Fixes #5991

Describe the solution (The How)

1. In construction.cpp, set construct::done_deconstruct to bother checking whether deconstruct placed t_null and if so, it looks for a potential roof assigned to the terrain on the z-level below it to use instead via get_roof, which in the case of stairs should create open air if there is a matching bottom stair below it.
2. Additionally set it so that construct::done_deconstruct checks whether the tile it would normally dump deconstruction items into would be valid (i.e. can hold items and isn't empty space), and if it isn't valid it'll pick a random valid space next to it in the same way construction byproducts are placed. Idea here is this stops it from spawning the deconstruct results directly over the hole that removing the stairs creates, which would promptly fall through and impact the bottom.
3. In map.h, moved get_roof from private to public because it wouldn't let me use the hidden gloryhole "access private member" in a different file without making it public.

Describe alternatives you've considered

1. Rigging t_null to automatically convert itself to the underlying roof anytime some flavor of fuckery causes it to accidentally spawn.
2. Making Coolthulhu cry by removing the bit in the code that makes the game complain if you explicitly try to set a deconstruct to create open air, which is what's blocking Ari's fix.

Testing

1. Compiled and load-tested.
2. Spawned in some wooden stairs across two z-levels.
3. Deconstructed the upper stairs, it now correctly turns into open air instead of void. Deconstruct results also no longer drop down the hole and cause a ruckus due to landing on the stairs below, instead being spawned under a random adjacent space.
4. Repeated test with the bottom stairs replaced with a solid wall, deconstructing the upper stairs instead correctly leaves behind a roof tile. Confirmed that items also still spawn on the usual tile since it creates normal floor to dump it on.
5. Deconstructed some furniture to confirm nothing weird happens, also works sanely if deconstructing furniture embedded in a wall.
6. Checked affected files for astyle.

Additional context

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

[RobbieNeko]

1. Looked over the code, don't see anything that looks off
2. Compiled
3. Ran the stair tests described in PR, confirmed they worked just fine for me too

Looks good to me!