Make null defaults nullable in Avro schema

[PhotonData-oss]

Summary

• convert fields with default=None to Avro nullable union types
• avoid double-wrapping types that are already nullable
• preserve default: null for optional/null-defaulted Pydantic fields
• add a regression test for the reported boolean/null-default schema

Verification

• git diff --cached --check
• python -m pytest tests/test_schema.py -q using a local virtualenv with Pydantic 1.10.26

Full pytest collection was not completed in this Python 3.11 environment because unrelated tests require missing optional dependencies (tqdm, confluent_kafka) and tests/test_models.py imports Iterator from collections, which is not available on Python 3.11.

Fixes #149