RFR: 8335294: Fix simple -Wzero-as-null-pointer-constant warnings in gc code

[Kim Barrett]

On Fri, 28 Jun 2024 12:37:41 GMT, Julian Waters <jwaters at openjdk.org> wrote:

> I would keep the return nullptr in that one place that it was removed (I'm somewhat certain that leaving out a return in a method that has a non void return type isn't allowed in C++, even if there was a call to a noreturn method directly before the return), but besides that this seems ok

Having execution run off the end of a function not returning void is UB.  Ending with a [[noreturn]] function prevents
that from happening (assuming the [[noreturn]] attribute is correctly applied, else that is itself UB).

We should instead be removing these "spurious" continuations / returns as encountered, to eliminate any questions
about whether, for example, a function _can_ return null.  I don't recommend going on a hunt to clean these up, as
that would likely be a lot of code churn.

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PR Comment: https://git.openjdk.org/jdk/pull/19934#issuecomment-2197755214