RFR: 8336085: Fix simple -Wzero-as-null-pointer-constant warnings in CDS code

[Julian Waters]

On Wed, 10 Jul 2024 12:00:02 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change that replaces some uses of literal 0 as a
> null pointer constant in cds code to instead use nullptr.
> 
> Testing: mach5 tier1

Marked as reviewed by jwaters (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/20108#pullrequestreview-2170925975