<i18n dev> RFR: 8039165: [Doc] MessageFormat null locale generates NullPointerException

[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 wording is specifically chosen as 'may throw' since whether an NPE is thrown depends on the subformat used by MessageFormat (see test example of construction with null locale and no exception thrown).

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

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Commit messages:
 - Clarify called vs invoked terminology
 - Add case of DoesNotThrow()
 - Spec and test change

Changes: https://git.openjdk.org/jdk/pull/14911/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14911&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8039165
  Stats: 149 lines in 3 files changed: 85 ins; 64 del; 0 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