RFR: 8177418: TimeZone.getTimeZone(String id) throws NullPointerException when id is null

[Justin Lu]

When ID is null, TimeZone.getTimeZone(String ID) throws a NullPointerException.

For example, 


String someID = null;
TimeZone tz1 = TimeZone.getTimeZone(someID);
``` 

throws a `NullPointerException`

This change updates the documentation to make this apparent.

-------------

Commit messages:
 - Add missing @throws

Changes: https://git.openjdk.org/jdk/pull/11888/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11888&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8177418
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11888.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11888/head:pull/11888

PR: https://git.openjdk.org/jdk/pull/11888