RFR: 8364984: Many jpackage tests are failing on Linux after JDK-8334238

[Alexey Semenyuk]

Fix NPE when `path.getParent()` in `LinuxHelper.getDesktopFiles()` returns null. The issue is deb-specific, there is no such problem with rpm packages.

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Commit messages:
 - 8364984: Many jpackage tests are failing on Linux after JDK-8334238

Changes: https://git.openjdk.org/jdk/pull/26677/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26677&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364984
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26677.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26677/head:pull/26677

PR: https://git.openjdk.org/jdk/pull/26677