RFR: 8326487: ZipFileSystem.getPath("").getFileName() returns null instead of an empty Path

[Jaikiran Pai]

Can I please get a review of this change which proposes to fix the issue reported in https://bugs.openjdk.org/browse/JDK-8326487?

`java.nio.file.Path.getFileName()` is specified to return a non-null value for an empty path. However, the current implementation of zipfs returns null for an empty path. The commit in this PR addresses that issue in zipfs and introduces a test which reproduces the issue and verifies the fix.

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

Commit messages:
 - include a test for root path
 - 8326487: ZipFileSystem.getPath().getFileName() returns null instead of an empty string

Changes: https://git.openjdk.org/jdk/pull/29760/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326487
  Stats: 46 lines in 2 files changed: 40 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/29760.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29760/head:pull/29760

PR: https://git.openjdk.org/jdk/pull/29760