RFR: 8374444: Fix simple -Wzero-as-null-pointer-constant warnings

[Kim Barrett]

Please review this trivial change to fix several recent changes that trigger
`-Wzero-as-null-pointer-constant `warnings when that warning is enabled.

Testing: mach5 tier1

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Commit messages:
 - fix simple -Wzero-as-null-pointer-constant warnings

Changes: https://git.openjdk.org/jdk/pull/29014/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29014&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374444
  Stats: 12 lines in 5 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/29014.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29014/head:pull/29014

PR: https://git.openjdk.org/jdk/pull/29014