RFR: 8338019: Fix simple -Wzero-as-null-pointer-constant warnings in riscv code

[Ludovic Henry]

On Thu, 8 Aug 2024 03:37:06 GMT, Gui Cao <gcao at openjdk.org> wrote:

> Hi, 
> Same as  [JDK-8337786](https://bugs.openjdk.org/browse/JDK-8337786) for aarch64, similar build warnings exist on riscv. Please help review this trivial change that replaces some uses of literal 0 as a null pointer constant in riscv code to instead use nullptr.
> 
> ### Testing
> - [x] Run tier1 tests on SOPHON SG2042 (release)

Marked as reviewed by luhenry (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/20506#pullrequestreview-2227338006