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

[Dean Long]

On Wed, 26 Feb 2025 15:11:25 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> 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.

Marked as reviewed by dlong (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23804#pullrequestreview-2645767681