RFR: 8325910: Rename jnihelper.h

[Kim Barrett]

Please review this trivial change that renames the file
test/hotspot/jtreg/vmTestbase/nsk/stress/jni/jnihelper.h to jnihelper.hpp, and
replaces uses of NULL in that file.

Testing: mach5 tier1

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Commit messages:
 - rename NULLs in jnihelper.hpp
 - rename jnihelper.hpp

Changes: https://git.openjdk.org/jdk/pull/17861/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17861&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325910
  Stats: 15 lines in 9 files changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/17861.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17861/head:pull/17861

PR: https://git.openjdk.org/jdk/pull/17861