RFR: 8335667: Fix simple -Wzero-as-null-pointer-constant warnings in compiler code

[Christian Hagedorn]

On Thu, 4 Jul 2024 09:17:46 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 compiler code to instead use nullptr.
> 
> Testing: mach5 tier1

Looks good and trivial.

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20023#pullrequestreview-2158664285