RFR: 8337418: Fix -Wzero-as-null-pointer-constant warnings in prims code

[Kim Barrett]

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

Testing: mach5 tier1

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Commit messages:
 - fix warnings in prims

Changes: https://git.openjdk.org/jdk/pull/20385/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20385&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337418
  Stats: 26 lines in 7 files changed: 0 ins; 3 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/20385.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20385/head:pull/20385

PR: https://git.openjdk.org/jdk/pull/20385