RFR: 8297814: (fs) Re-visit Path.getExtension return value

[Brian Burkhalter]

Revise `java.nio.file.Path::getExtension` never to return `null` and to simplify implementing other path manipulations such as removing and replacing the file name extension.

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Commit messages:
 - 8297814: (fs) Re-visit Path.getExtension return value

Changes: https://git.openjdk.org/jdk/pull/11545/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11545&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297814
  Stats: 70 lines in 2 files changed: 10 ins; 8 del; 52 mod
  Patch: https://git.openjdk.org/jdk/pull/11545.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11545/head:pull/11545

PR: https://git.openjdk.org/jdk/pull/11545