RFR: 8324680: Replace NULL with nullptr in JVMTI generated code

[Serguei Spitsyn]

This enhancement replaces uses of NULL with nullptr in the XML-description files for JVMTI. These are the files `hotsport/share/prims/jvmti.xml` and `hotspot/share/prims/jvmti*.xls`.

The following files are auto-generated from the `jvmti.xml` and `*.xsl files` in the `build/<CFG>/variant-server/gensrc/jvmtifiles': `jvmti.h`, `jvmti.html`, `jvmtiEnter.cpp`, `jvmtiEnterTrace.cpp`, `jvmtiEnv.hpp`

This addresses a category of NULL uses that wasn't dealt with by:
 [JDK-8299837](https://bugs.openjdk.org/browse/JDK-8299837).
 
 Testing:
   - TBD: run mach5 tiers1-3

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Commit messages:
 - 8324680: Replace NULL with nullptr in JVMTI generated code

Changes: https://git.openjdk.org/jdk/pull/17866/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324680
  Stats: 110 lines in 5 files changed: 0 ins; 0 del; 110 mod
  Patch: https://git.openjdk.org/jdk/pull/17866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17866/head:pull/17866

PR: https://git.openjdk.org/jdk/pull/17866