RFR: 8337243: Fix more -Wzero-as-null-pointer-constant warnings in compiler code [v2]

[Vladimir Ivanov]

On Fri, 26 Jul 2024 03:55:05 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
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove extraneous whitespace

Marked as reviewed by vlivanov (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/20343#pullrequestreview-2200979758