v2.1.0: Added 'Only' helper and fixed an issue with empty mappings

This release contains a fix for null mapping - They were previously erroring out when a @Key UDA was used.

What's Changed

• feat(attributes): Add a simple 'Only' helper to help with enums by @Geod24 in #68
• fix(read): Improve handling of Key UDA for empty mapping (backport of #82)

Full Changelog: v2.0.0...v2.1.0