RFR: 8337243: Fix more -Wzero-as-null-pointer-constant warnings in compiler code

[Kim Barrett]

Please review this (perhaps trivial?) change that removes some uses of literal
0 as a null pointer constant in compiler code. Most are changed to use nullptr.
There are a couple where the better approach is to use a value-initialized
object of the appropriate template parameter type.

Testing: mach5 tier1

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

Commit messages:
 - fix simple compiler

Changes: https://git.openjdk.org/jdk/pull/20343/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20343&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337243
  Stats: 12 lines in 7 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20343.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20343/head:pull/20343

PR: https://git.openjdk.org/jdk/pull/20343