[lworld] RFR: 8322573: [lworld] Add null-restricted value class array support in core reflection

[Mandy Chung]

This patch fixes `Array::set` that if `null` is set on an element of a null-restricted arra, NPE will be thrown.

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Commit messages:
 - test clean up
 - 8322573: [lworld] Add null-restricted value class array support in core reflection

Changes: https://git.openjdk.org/valhalla/pull/965/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=965&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322573
  Stats: 103 lines in 3 files changed: 64 ins; 5 del; 34 mod
  Patch: https://git.openjdk.org/valhalla/pull/965.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/965/head:pull/965

PR: https://git.openjdk.org/valhalla/pull/965