What

UpdateItem with only TableName and Key (no UpdateExpression, no AttributeUpdates) now matches DynamoDB:

• Missing item: HTTP 200, key-only item upserted.
• Existing item: HTTP 200, no-op.

Empty UpdateExpression ("") still rejected via tokenize_for (preserves PR #24, commit ef8b94f).

Why

Closes #118

Testing done

Tested against real DynamoDB to confirm the expected behavior, then verified the fix against ExtendDB end-to-end. New tests:

• 3 unit tests on validate_update_item covering None, Some({}), and Some("").
• 2 pytest tests in TestUpdateItem covering the missing-item upsert and existing-item no-op.

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace)
• Code is formatted (cargo fmt --check)
• Clippy is clean (cargo clippy --all-targets --workspace -- -D warnings)
• I have added or updated tests for new functionality
• I have updated documentation if behavior changed
• Breaking changes are noted below (if any)

Breaking changes

None.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache License 2.0 and I agree to the Developer Certificate of Origin (DCO). See CONTRIBUTING.md for details.