RFR: 8345505: Fix -Wzero-as-null-pointer-constant warnings in zero code

[Kim Barrett]

Please review this trivial change to remove some -Wzero-as-null-pointer-constant
warnings in zero code.  Just replaced two literal 0's with nullptr.

Testing: mach5 tier1, GHA sanity tests

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Commit messages:
 - fix warnings in zero

Changes: https://git.openjdk.org/jdk/pull/22728/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345505
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22728.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22728/head:pull/22728

PR: https://git.openjdk.org/jdk/pull/22728