RFR: 8335663: Fix simple -Wzero-as-null-pointer-constant warnings in C2 code

[Julian Waters]

On Thu, 4 Jul 2024 07:06:39 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 C2 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/20022#pullrequestreview-2158162219