Simplify and deduplicate PaperweightGetBlocks#update logic

[SirYwell]

Overview

Description

We can simplify the logic in the update method:

• Avoid filling the array if all of its content is written later anyway
• Move version-independent loops to common supertype
• Use Arrays.fill instead of manual loops where possible

This also helps avoiding going through hard-to-optimize initialization check code every iteration instead of only once before

Submitter Checklist

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