RFR: 7038838: Unspecified NPE in java.net.IDN methods

[Jaikiran Pai]

Can I please get a review of this doc-only changes which specifies the methods on `java.net.IDN` to throw `NullPointerException` when any argument is null?

This addresses https://bugs.openjdk.org/browse/JDK-7038838. As noted in that issue, the current implementation of these methods already throw a NullPointerException for any null argument. This change now updates the specification to match the current implementation. 

A new jtreg test has been added to verify this behaviour. Note that there already was a `test/jdk/java/net/Socket/IDNTest.java` test but looking what that test does, the history of that test and the location of that test in `java/net/Socket` directory made me believe that it's not meant for testing the `java.net.IDN` class. That's why I created this new test class in `test/jdk/java/net/` directory.

I'll draft a CSR for this change.

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Commit messages:
 - 7038838: Unspecified NPE in java.net.IDN methods

Changes: https://git.openjdk.org/jdk/pull/22463/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22463&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7038838
  Stats: 51 lines in 2 files changed: 51 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22463.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22463/head:pull/22463

PR: https://git.openjdk.org/jdk/pull/22463