RFR: 8337239: Fix simple -Wzero-as-null-pointer-constant warnings in classfile code

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a
null pointer constant in classfile code to instead use nullptr.

Testing: mach5 tier1

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

Commit messages:
 - simple fixes in classfiles

Changes: https://git.openjdk.org/jdk/pull/20340/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20340&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337239
  Stats: 16 lines in 4 files changed: 0 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/20340.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20340/head:pull/20340

PR: https://git.openjdk.org/jdk/pull/20340