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

[Kim Barrett]

On Thu, 4 Jul 2024 11:05:13 GMT, Christian Hagedorn <chagedorn 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.

Thanks for review @chhagedorn

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PR Comment: https://git.openjdk.org/jdk/pull/20023#issuecomment-2208827904