<i18n dev> [jdk21] RFR: 8039165: [Doc] MessageFormat null locale generates NullPointerException

[Justin Lu]

Please review this change which contains a backport of commit [c6396dce](https://github.com/openjdk/jdk/commit/c6396dceb9a64578d5b335af27ad1d968190a1fa) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

This PR is a java.text doc/test-only change.

The commit being backported was authored by Justin Lu on 25 Jul 2023 and was reviewed by Naoto Sato.

Thanks!

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Commit messages:
 - Backport c6396dceb9a64578d5b335af27ad1d968190a1fa

Changes: https://git.openjdk.org/jdk21/pull/154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8039165
  Stats: 164 lines in 3 files changed: 97 ins; 64 del; 3 mod
  Patch: https://git.openjdk.org/jdk21/pull/154.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/154/head:pull/154

PR: https://git.openjdk.org/jdk21/pull/154