<i18n dev> RFR: 8351074: Disallow null prefix and suffix in DecimalFormat

[Justin Lu]

Please review this PR and associated CSR which disallows passing null to 4 `DecimalFormat` prefix/suffix setter methods.

Currently these setters do not check the input String for null. When the prefix/suffix is null, any such DecimalFormat instances are effectively non-functional as it will throw NPE for most operations.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/23880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351074
  Stats: 60 lines in 2 files changed: 59 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23880/head:pull/23880

PR: https://git.openjdk.org/jdk/pull/23880