RFR: 8336346: Fix -Wzero-as-null-pointer-constant warnings in jvmciJavaClasses.cpp [v2]

[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

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  fix backslash alignment in jvmciJavaClasses change

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20175/files
  - new: https://git.openjdk.org/jdk/pull/20175/files/c6734072..d5b51135

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20175&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20175&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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