fix: support None custom properties

[Ramlaoui]

Summary

• add an explicit null property type to the Rust storage model and SOA schema
• accept and return None in the Python property bridge and database readers
• preserve None values when importing custom properties from ASE metadata

Testing

• rtk cargo test -p atompack soa_all_property_value_types
• rtk uv run --no-sync pytest tests/test_atom_molecule.py::test_molecule_custom_properties tests/test_database.py::test_database_roundtrip -q
• rtk uv run --no-sync python -c ... (from_ase nullable property smoke check)

Closes #4