RFR: 8377910: Minor cleanup of java/io/FileDescriptor/Sharing.java

[Brian Burkhalter]

Null checks potentially mask a more serious, fundamental problem and should be removed to allow a `NullPointerException` to be throw if the variable is `null`.

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Commit messages:
 - 8377910: Minor cleanup of java/io/FileDescriptor/Sharing.java

Changes: https://git.openjdk.org/jdk/pull/29718/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377910
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/29718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29718/head:pull/29718

PR: https://git.openjdk.org/jdk/pull/29718