RFR: 8326716: JVMTI spec: clarify what nullptr means for C/C++ developers

[Serguei Spitsyn]

The following RFE was fixed recently:
[8324680](https://bugs.openjdk.org/browse/JDK-8324680): Replace NULL with nullptr in JVMTI generated code

It replaced all the `NULL`'s in the generated spec with`nullptr`. JVMTI agents can be developed in C or C++.
This update is to make it clear that `nullptr` is C programming language `null` pointer.

I think we do not need a CSR for this fix.

Testing: N/A (not needed)

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Commit messages:
 - 8326716: JVMTI spec: clarify what nullptr means for C/C++ developers

Changes: https://git.openjdk.org/jdk/pull/19257/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19257&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326716
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19257.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19257/head:pull/19257

PR: https://git.openjdk.org/jdk/pull/19257