RFR: 8345492: Fix -Wzero-as-null-pointer-constant warnings in adlc code

[Kim Barrett]

Please review this trivial change to adlc, to use nullptr instead of literal 0
as a null pointer constant.

Testing: mach5 tier1
Locally tested (linux-x64) with -Wzero-as-null-pointer-constant enabled to
verify the warnings associated with this code were removed.

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Commit messages:
 - fix warnings in adlc

Changes: https://git.openjdk.org/jdk/pull/23804/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23804&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345492
  Stats: 9 lines in 2 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/23804.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23804/head:pull/23804

PR: https://git.openjdk.org/jdk/pull/23804