RFR: 8300731: Avoid unnecessary array fill after creation in PaletteBuilder

[Andrey Turbanov]

No need to fill elements of array with `null`: it was just created. Java guarantees that all elements of array are `null`s anyway.

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Commit messages:
 - [PATCH] Avoid unnecessary array fill after creation

Changes: https://git.openjdk.org/jdk/pull/12101/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300731
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12101.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12101/head:pull/12101

PR: https://git.openjdk.org/jdk/pull/12101