RFR: 8337523: Fix -Wzero-as-null-pointer-constant warnings in jvmci code

[Kim Barrett]

Please review this trivial change that removes some uses of literal 0 as a
null pointer constant in jvmci code.

Testing: mach5 tier1

-------------

Commit messages:
 - fix simple jvmci

Changes: https://git.openjdk.org/jdk/pull/20399/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20399&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337523
  Stats: 6 lines in 4 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20399.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20399/head:pull/20399

PR: https://git.openjdk.org/jdk/pull/20399