Simplify NMSAdapter#createPalette methods

[SirYwell]

Overview

Description

There are a few things we can simplify here:

• The short[] nonEmptyBlockCount was always null, so we can remove it
• As a consequence, we don't need to calculate non-air block count anymore
• We can use int[] instead of char[] for the global palette mapping, and use System.arraycopy then
• We can extract duplicated code of the two variants into a common method

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