RFR: 8346008: Fix recent NULL usage backsliding in Shenandoah

[Sonia Zaldana Calles]

Hi all, 

This PR addresses [8346008](https://bugs.openjdk.org/browse/JDK-8346008). It's a follow-up from [8345647](https://bugs.openjdk.org/browse/JDK-8345647) with some cases I missed. 

Thanks, 
Sonia

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Commit messages:
 - 8346008: Fix recent NULL usage backsliding in Shenandoah

Changes: https://git.openjdk.org/jdk/pull/22684/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22684&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346008
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22684.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22684/head:pull/22684

PR: https://git.openjdk.org/jdk/pull/22684