fix(output): preserve explicit empty string fields during TSV conversion

[Athemis]

Summary

This PR fixes #29 a TSV conversion bug where explicit empty strings in YAML (for example watermark: "") were treated as invalid entries.

Before this change, convert printed Invalid entry '' and could shift subsequent fields in affected TSV rows.

Root Cause

In yml2block/output.py, explicit empty strings ("") were not handled as valid values.
They fell through to the error branch instead of being written as an empty TSV cell.

Changes

• Fix: Treat value == "" as a valid empty output cell during TSV writing.
• Test: Added regression coverage for this behavior in integration tests.
• Fixture: Added tests/valid/explicit_empty_string_watermark.yml to match existing test data conventions.

Files Changed

• yml2block/output.py
• tests/integration_tests.py
• tests/valid/explicit_empty_string_watermark.yml

Verification

Ran:

poetry run pytest -q tests/unit_tests.py tests/integration_tests.py

Result:

• 19 passed

Additionally re-ran conversion on the reported real-world input:

• No more Invalid entry '' messages
• No column shifts in the previously affected rows

[HenningTimm]

Thank you! Both for the fix and the extensive documentation :)