RFR: 8337243: Fix more -Wzero-as-null-pointer-constant warnings in compiler code

[Vladimir Ivanov]

On Thu, 25 Jul 2024 23:59:54 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this (perhaps trivial?) change that removes some uses of literal
> 0 as a null pointer constant in compiler code. Most are changed to use nullptr.
> There are a couple where the better approach is to use a value-initialized
> object of the appropriate template parameter type.
> 
> Testing: mach5 tier1

Looks good and trivial.

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Marked as reviewed by vlivanov (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20343#pullrequestreview-2200643262