[MC-125138] Mesa surface mineshafts place cave_air even if mineshaft piece did not place blocks

Mesa surface mineshafts always cave_air, even if the mineshaft piece did not place any blocks because it would be floating, see (18w06a) Mesa mineshafts generating cave_air without mineshaft blocks.png.

Create a world with the following seed
```
-9144014475768710727
```
Teleport to the mineshaft
```
/teleport -126 90 162
```
Download the attached datapack cave_air marker datapack.zip and place it in the datapacks folder of the world
Reload the datapacks
Use the provided function to mark cave_air blocks
```
/function marcono1234:replace_cave_air
```