RFR: 8337418: Fix -Wzero-as-null-pointer-constant warnings in prims code

[Serguei Spitsyn]

On Tue, 30 Jul 2024 04:12:33 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

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

Looks good. Thank you for fixing this!
The `ResultType ret{};` syntax is a little bit unusual but I'm okay with that. :)

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Marked as reviewed by sspitsyn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20385#pullrequestreview-2209490825