fix: preserve empty query string values - #187

Summary

• omit only None scalar query values
• preserve supplied empty strings as key=
• cover top-level, nested, and repeated-array empty values

Why

The serializer converted an empty string to "" and then treated that valid serialized value as absent. APIs can distinguish a missing query key from a present-but-empty key, so callers need to be able to express both.

Fixes #179

Testing

• python -m pytest tests/test_qs.py -q (11 passed)
• python -m mypy src/browserbase/_qs.py
• python -m pyright src/browserbase/_qs.py tests/test_qs.py
• python -m ruff check src/browserbase/_qs.py tests/test_qs.py
• python -m ruff format --check src/browserbase/_qs.py tests/test_qs.py
• full suite: 1217 passed, 8 skipped; 4 pre-existing Windows/environment failures tracked in Make the test suite deterministic on Windows and proxied environments #181