RFR: 8327171: Fix more NULL usage backsliding

[Julian Waters]

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

Marked as reviewed by jwaters (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/18096#pullrequestreview-1912975362