RFR: 8336346: Fix -Wzero-as-null-pointer-constant warnings in jvmciJavaClasses.cpp

[Kim Barrett]

Please review this trivial change to remove -Wzero-as-null-pointer-constant
warnings (when enabled), triggered by assignment to or comparison with a
jfieldID (a pointer type) with a value of 0.  Changing the value to nullptr
removes the warnings.

Because these changes are in macros that are used many times, this change
removes about 100 warnings.

Testing: mach5 tier1

-------------

Commit messages:
 - fix jvmciJavaClasses macros

Changes: https://git.openjdk.org/jdk/pull/20175/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336346
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20175.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20175/head:pull/20175

PR: https://git.openjdk.org/jdk/pull/20175