test(dtype): cover unicode fields in the structured default scalar test

[d-v-b]

🤖 AI text below 🤖

Structured data types now default string (S, U) and raw-bytes (V) fields to zero bytes. Before, an array with a raw-bytes field failed to create without an explicit fill value, and string fields defaulted to "0".

Summary

This is a follow-up to #4495. Moving from np.array([0], dtype)[0] to np.zeros(1, dtype)[0] also changed the default of U fields from '0' to '', but the test and the changelog entry only named S and V. This PR adds U4 to the parametrized test and rewords the unreleased changes/4495.bugfix.md entry.

For reviewers

There is no new fragment, because the behavior change already ships under #4495. The agent checked that all three test cases fail on the parent of #4495 and pass on main. No library code changes.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions (no API change)
• New/modified features documented in docs/user-guide/*.md (no feature change)
• Changes documented as a new file in changes/ (edits the unreleased 4495.bugfix.md instead)
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

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[d-v-b]

teeny follow-up to #4495