<i18n dev> RFR: 8369452: Locale.Builder.setLanguageTag(String) does not clear on empty or null String

[Justin Lu]

Please review this PR which corrects the behavior of `Locale.Builder.setLanguageTag(String)` on the null and empty String case.

> Null and the empty string cause the builder to be reset

Currently, NPE is thrown for a null tag, and ILE is thrown for an empty String. They are both corrected to not throw, and instead reset the state of the `Locale.Builder`.

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Commit messages:
 - The fix and reg test

Changes: https://git.openjdk.org/jdk/pull/27734/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27734&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369452
  Stats: 25 lines in 2 files changed: 20 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/27734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27734/head:pull/27734

PR: https://git.openjdk.org/jdk/pull/27734