RFR: 8337787: Fix -Wzero-as-null-pointer-constant warnings when JVMTI feature is disabled

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a
null pointer constant when JVMTI is disabled to instead use nullptr.

Testing: mach5 tier1

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Commit messages:
 - fix NOT_JVMTI

Changes: https://git.openjdk.org/jdk/pull/20455/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20455&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337787
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20455.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20455/head:pull/20455

PR: https://git.openjdk.org/jdk/pull/20455