Fix deadlock in NamespacedHierarchicalStore.computeIfAbsent()

[mpkorstanje]

Fixes: #5346

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[testlens-app]

✅ All tests passed ✅

🏷️ Commit: fdc46c3
▶️ Tests: 32687 executed
⚪️ Checks: 15/15 completed

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