Handle empty results in batch geocode responses

[gaoflow]

Summary

• skip batch geocode entries whose nested response.results list is empty
• keep parsing later successful entries in the same batch
• add a regression test for mixed valid and invalid address batches

Fixes #24.

Tests

• uv run --extra dev pytest tests/unit/test_geocode.py::test_geocode_batch_with_invalid_address_result -q
• uv run --extra dev pytest tests/unit -q
• uv run --extra dev black --check src/geocodio/client.py tests/unit/test_geocode.py
• uv run --extra dev isort --check-only src/geocodio/client.py tests/unit/test_geocode.py
• uv run --extra dev flake8 src/ --ignore=E501
• python -m compileall -q src/geocodio tests/unit/test_geocode.py
• git diff --check