RFR: 8337416: Fix -Wzero-as-null-pointer-constant warnings in misc. runtime code

[Kim Barrett]

Please review this (perhaps trivial?) change that removes some uses of literal
0 as a null pointer constant in misc. runtime code.  Most are changed to use
nullptr. 

Testing: mach5 tier1

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

Commit messages:
 - fix simple runtime

Changes: https://git.openjdk.org/jdk/pull/20383/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20383&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337416
  Stats: 17 lines in 10 files changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/20383.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20383/head:pull/20383

PR: https://git.openjdk.org/jdk/pull/20383