RFR: 8345647: Fix recent NULL usage backsliding in Shenandoah

[Sonia Zaldana Calles]

Hi all, 

This PR addresses [8345647](https://bugs.openjdk.org/browse/JDK-8345647) addressing uses of `NULL` in Shenandoah. These should be replaced to `null`. 

I verified where those uses are with: 
`git show <commit-id> --name-only | while read file; do echo "File: $file"; git diff <commit-id> -- "$file" | grep -n -F 'NULL'; done`

The relevant output: 

File: src/hotspot/cpu/ppc/gc/shenandoah/shenandoahBarrierSetAssembler_ppc.cpp
9:-  // No need for post barrier if storing NULL
File: src/hotspot/share/gc/shenandoah/c2/shenandoahBarrierSetC2.cpp
9:-    // must be either an oop or NULL
12:     if (t == TypePtr::NULL_PTR || t == Type::TOP)


Changes affect comments only. 

Thanks, 
Sonia

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Commit messages:
 - 8345647: Fix recent NULL usage backsliding in Shenandoah

Changes: https://git.openjdk.org/jdk/pull/22647/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22647&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345647
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22647.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22647/head:pull/22647

PR: https://git.openjdk.org/jdk/pull/22647