Fix issue 567 conditional save default

[ljluestc]

Fixes #567, where conditional saves fail for existing items when a new field with a default value is added to a model. The issue occurs because existing items have null or missing values, causing a mismatch during conditional saves.

Changes

• Modified Model.save to apply default values for null attributes before saving.
• Added update_items_with_default_values utility to migrate existing items.
• Added tests in tests/test_conditional_save.py to verify the fix.
• Updated documentation in docs/guide.rst and CHANGELOG.md.

Testing

• Added async tests using pytest-asyncio to cover both existing and new items.
• Verified with local DynamoDB table and AWS credentials.
• Ran black and flake8 to ensure code style compliance.

Closes #567.