RFR: 8356450: NPE in CLDRTimeZoneNameProviderImpl for tzdata downgrades after JDK-8342550

[Naoto Sato]

Fixing the NPE in CLDR time zone name provider. The NPE occurrs if the time zone data was downgraded by the TZUpdater tool in which some time zones are missing. For those missing zones, `ZoneInfo.getTimeZone()` returns null, while `TimeZone.getTimeZone()` falls back to `GMT`, which was the case prior to JDK-8342550. Changed the code in CLDR provider to assume the fixed zone in such a case.
Manually confirmed the fix, and no test case is provided, as it requires tweaking the JDK with TZUpdater.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/25130/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356450
  Stats: 10 lines in 1 file changed: 7 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25130.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25130/head:pull/25130

PR: https://git.openjdk.org/jdk/pull/25130