RFR: 8350771: Fix -Wzero-as-null-pointer-constant warning in nsk/monitoring ThreadController utilitiy

[Kim Barrett]

Please review this trivial change to remove a use of literal zero as a null
pointer constant in an nsk test utility.

Testing: mach5 tier1
Locally tested (linux-x64) with -Wzero-as-null-pointer-constant enabled to
verify the warnings associated with this code were removed.

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Commit messages:
 - fix nsk/monitoring ThreadController.cpp

Changes: https://git.openjdk.org/jdk/pull/23800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350771
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23800/head:pull/23800

PR: https://git.openjdk.org/jdk/pull/23800