fix: ThreadUnsafeCharBlocks#reset using clear without null check

[Timongcraft]

Overview

I discovered this while rewriting my logic that captures a paste and prevents a world flush.
This is a minimal fix I can also change it to clear conditionally and mark the maps with MonotonicNonNull or Nullable.
CharSetBlocks's reset also null assigns the biomes etc., which could also be done here.

Description

Replace the map clear calls in ThreadUnsafeCharBlocks with null assignment to allow resetting chunk sets without all kinds of data such as tile entities and entities.

Submitter Checklist

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