<i18n dev> RFR: 8366401: JCK test api/java_text/DecimalFormatSymbols/serial/InputTests.html fails after JDK-8363972

[Justin Lu]

This PR addresses a JCK test failure related to `DecimalFormatSymbols` de-serialization. While the current public API of DFS disallows a null locale, it was possible to set in the past. Thus, the `loadNumberData(locale)` call currently throws NPE when locale is null in the stream. The call should be guarded with a null check, such that if locale is null, then `lenientMinusSigns` defaults to `minusSignText`.

Defaulting the locale field when `null` to Locale.ROOT is also a reasonable solution, but I think that the current one is preferable as a user would not expect locale data related logic to occur if locale is `null`.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/27008/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27008&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366401
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27008.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27008/head:pull/27008

PR: https://git.openjdk.org/jdk/pull/27008