Avoid NullPointerException when deserializing TestIdentifier

[dmlloyd]

Overview

An NPE was thrown when (de)serializing TestIdentifiers without a parent.

Fixes #3819.

This is a backport of #3820 to the 5.10.x branch.

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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

[holly-cummins]

I think this PR can be closed because of d29e3eb (and #3838).

[marcphilipp]

You're right! Thanks for the pointer.