- Fixed test_extract_combo_with_multi_select_flag: combo boxes are always single-select regardless of multi-select flag
- Fixed test_extract_default_none_becomes_none: empty string defaults are valid and should not be filtered out
- Added is_truly_empty() method to distinguish between no value (None) and empty string value
- Updated verification note for pdftract-5t92
Refs: pdftract-5t92, plan 7.4.2
