RFR: 8375737: Fix -Wzero-as-null-pointer-constant warnings in arm32 code

[Kim Barrett]

Please review this trivial change to arm32 code to avoid using literal zero as a null
pointer constant, instead using nullptr.

Testing: GHA build for arm32

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Commit messages:
 - fix arm32 -Wzero-as-null-pointer-constant warnings

Changes: https://git.openjdk.org/jdk/pull/29328/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29328&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375737
  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/29328.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29328/head:pull/29328

PR: https://git.openjdk.org/jdk/pull/29328