RFR: 8337243: Fix more -Wzero-as-null-pointer-constant warnings in compiler code [v2]

[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

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

  remove extraneous whitespace

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20343/files
  - new: https://git.openjdk.org/jdk/pull/20343/files/8cd0bc25..f1242f4f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20343&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20343&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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