RFR: 8303227: JniObjWithEnv should be NullablePointer compliant

[Julian Waters]

On Mon, 27 Feb 2023 08:19:53 GMT, Julian Waters <jwaters at openjdk.org> wrote:

> JniObjWithEnv is a struct that is commonly managed by std::unique_ptr. Although it can support managing objects that are not raw pointers, any such objects have to be [NullablePointers](https://en.cppreference.com/w/cpp/named_req/NullablePointer). In the past this has [broken the build when compiler upgrades were carried out](https://bugs.openjdk.org/browse/JDK-8244220), so we should add in the final requirements to make the struct a true NullablePointer once and for all, to prevent similar issues from happening in the future

@alexeysemenyukoracle Sorry for the ping, just wanted to avoid the PR getting buried since I don't know any other jpackage reviewers

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PR: https://git.openjdk.org/jdk/pull/12758