RFR: 8039165: [Doc] MessageFormat null locale generates NullPointerException [v4]

[Justin Lu]

> Please review this PR and [CSR](https://bugs.openjdk.org/browse/JDK-8312197) which updates the javadoc for the constructor of MessageFormat regarding a `null` locale,
> 
> `MessageFormat` when created with a `null` locale may throw a `NullPointerException` either during the object creation, or later when `format()` is called by the `MessageFormat` object (test file has examples of both). This change updates the specification of MessageFormat to make this apparent.
> 
> The test for this change was merged with `Bug6481179.java` into `MessageFormatExceptions.java` (As they both test exceptions). In addition, some other exception testing regarding MessageFormat was added.
> 
> Thanks

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Account for setLocale()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14911/files
  - new: https://git.openjdk.org/jdk/pull/14911/files/8fd94758..b075f5d0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14911&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14911&range=02-03

  Stats: 10 lines in 2 files changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14911.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14911/head:pull/14911

PR: https://git.openjdk.org/jdk/pull/14911