[UmutAlisoglu]

Describe your changes

An empty-string parent key is treated as the root prefix during flattening. This loses part of the path and can raise a false conflict:

benedict({"": {"child": 1}, "child": 2}).flatten()
# before: KeyError for 'child'
# after: {'_child': 1, 'child': 2}

Use None as the internal root-prefix sentinel so an actual empty-string key still contributes a separator. Extend the existing falsy-parent tests and add a public round trip covering nested empty keys, indexed lists and custom separators. Source data stays unchanged.

Related issue

No matching issue or PR found. This follows the falsy-parent fix in #600, whose scope explicitly left empty-string behavior unchanged. The reproduction above demonstrates the remaining case.

Validation

• Original source with the four affected test methods: 3 failing assertions and 5 errors, all in the new empty-key subcases.
• Windows / Python 3.14.7: full suite, 991 tests passed. S3 credentials were empty, so no real S3 operations are claimed.
• After final formatting: focused flatten/unflatten/public API suite, 114 tests passed; flatten.py has 100% line and branch coverage.
• Strict mypy passes on all 65 package files.
• All applicable pinned pre-commit hooks pass on the changed files.

Checklist before requesting a review

• I have performed a self-review of my code. (AI-assisted review; no human review claimed.)
• I have added tests for the proposed changes.
• I have run the tests and there are not errors.

AI assistance: OpenAI Codex assisted with investigation, implementation, review and verification.