RFR: 8350767: Fix -Wzero-as-null-pointer-constant warnings in nsk jni stress tests

[Kim Barrett]

Please review this change to remove uses of literal zero as a null pointer
constant in nsk jni stress tests.

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 warnings in vmTestbase/nsk/stress/jni

Changes: https://git.openjdk.org/jdk/pull/23798/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23798&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350767
  Stats: 20 lines in 5 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/23798.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23798/head:pull/23798

PR: https://git.openjdk.org/jdk/pull/23798