fix: Deconstructing stairs creates "nothing"

[arijust]

Purpose of change (The Why)

Close #5991

Describe the solution (The How)

Change t_null into t_open_air for its terrain setting in deconstruction.

Describe alternatives you've considered

None.

Testing

1. Teleport to location with wooden stairs.
2. Deconstruct them.
3. Teleport floor above around stairs.
4. You should see open air instead of void in reality.

Additional context

Checklist

Mandatory

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[scarf005]

https://github.com/cataclysmbnteam/Cataclysm-BN/actions/runs/13586048297/job/37981170421?pr=6141

 (~[slow] ~[.],starting_items)=> (continued from above) ERROR: t_wood_stairs_down deconstructs into "t_open_air", but "t_null" is preferred

looks like for some reason stairs are meant to deconstruct into t_null; code changes are necessary.