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

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`. ------------- 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