RFR: 8297814: (fs) Re-visit Path.getExtension return value [v2]

[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.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8297814: Address reviewer comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11545/files
  - new: https://git.openjdk.org/jdk/pull/11545/files/1c240293..af098065

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11545&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11545&range=00-01

  Stats: 29 lines in 2 files changed: 13 ins; 9 del; 7 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