RFR: 8337523: Fix -Wzero-as-null-pointer-constant warnings in jvmci code

[Aleksey Shipilev]

On Wed, 31 Jul 2024 02:12:39 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change that removes some uses of literal 0 as a
> null pointer constant in jvmci code.
> 
> Testing: mach5 tier1

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/20399#pullrequestreview-2209561186