RFR: 8327171: Fix more NULL usage backsliding

[Guoxiong Li]

On Sun, 3 Mar 2024 05:19:45 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change that replaces some recently introduced uses
> of NULL with nullptr.
> 
> Testing: mach5 tier1

Looks good.

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Marked as reviewed by gli (Committer).

PR Review: https://git.openjdk.org/jdk/pull/18096#pullrequestreview-1912954350