RFR: 8374441: FileSystemProvider.readAttributesIfExists should return null for ENOTDIR (unix)

[Fabian Meumertzheim]

`FileSystemProvider.readAttributesIfExists` now returns `null` for paths with a true prefix that is not a directory rather than throwing `NotDirectoryException`.

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Commit messages:
 - 8374441: FileSystemProvider.readAttributesIfExists should return null for ENOTDIR (unix)

Changes: https://git.openjdk.org/jdk/pull/29013/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29013&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374441
  Stats: 17 lines in 2 files changed: 16 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29013.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29013/head:pull/29013

PR: https://git.openjdk.org/jdk/pull/29013