tests: expect NULL _row_status for matched rows

[simbo1905]

Update test expectations to reflect that _row_status is now NULL when rows exist on both sides (previously 0).

• Adjust assertions in tests/test_diffgen.py to use None for _row_status in matched-row cases.
• Keep presence/absence statuses (4 missing in before, 5 missing in after) unchanged.
• Add/clarify comments in tests about the new semantics.

Refs #3.