RFR: 8303227: JniObjWithEnv should be NullablePointer compliant

[Julian Waters]

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

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Commit messages:
 - JniObjWithEnv should be NullablePointer compliant

Changes: https://git.openjdk.org/jdk/pull/12758/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12758&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303227
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12758.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12758/head:pull/12758

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