test(markdown): cover empty and mixed block property values

[MarcoPorcellato]

Context

The Logseq writer serializes standard block properties and LOGBOOK drawers. Edge cases for empty values and mixed list content are not specified by small tests.

GFI-61 — test only, estimated 1–3 hours.

Task

Add table-driven tests through format_logseq_block_property_lines for current empty and mixed-value behavior.

Suggested files

• tests/test_logseq_markdown.py
• Reference: src/logseq_matryca_parser/logseq_markdown.py

Acceptance criteria

• Empty scalar properties follow the current helper policy consistently.
• Empty list items do not create malformed lines.
• Strings, numbers, and booleans preserve their rendered values.
• Existing id and logbook behavior remains covered.

Out of scope

• Parser grammar changes
• New property types
• Changing serialization policy

Verify

uv run pytest tests/test_logseq_markdown.py -q
make all

No changelog entry is required because this is test-only.