fix: don't attempt to write structure void blocks in minecraft structure

[PierreSchwang]

Overview

Fixes #3090

### Submitter Checklist
- [x] Make sure you are opening from a topic branch (**/feature/fix/docs/ branch** (right side)) and not your main branch.
- [x] Ensure that the pull request title represents the desired changelog entry.
- [x] New public fields and methods are annotated with `@since TODO`.
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[dordsor21]

Change this top javax while we're here I guess?

[PierreSchwang]

That would be great, but i hate that

guess i could suppress that simply (edit: or wait, i can simply use a var assignment)

[dordsor21]

Like I said on Discord, I wonder if it's an idea to add a switch to allow/disallow structure void to be pasted - I can see it being useful to know where they are

[RedstoneFuture]

By default, Structure Void blocks should be pasted because they are simply part of the structure / schematic, I think.

In vanilla, structure_void is like the //paste command flag “-a” from FAWE, but “-a” affects air and structure_void would remain. So the vanilla behavior is exactly the opposite of the FAWE behavior when using "-a".

Maybe a paste command flag for FAWE would be an idea, which would paste AIR like in Vanilla and leave out structure_void blocks. Anything else would perhaps be unusual.

[Leomelonseeds]

Just ran into this issue saving Minecraft structure schematics for my plugin, would appreciate this getting merged!