* Fix null identity matching, string-based nested keys (v4)

- indexOfItemInArray/applyArrayChange/revertArrayChange: use !== undefined
  instead of != null so null identity values can match
- compareArray: set embeddedKeyIsPath for string keys with dots when data
  uses nested structure (not literal dot-property)
- convertArrayToObj: resolve nested paths for string identity keys
- Guard in operator against primitives
- Bump to 4.10.4

* Address review: robust sample element selection, add missing tests

- Use first non-null object element as sample for nested path detection
- Add test: string-based nested identity key with full round-trip
- Add test: null identity value diff + apply + atomize round-trip

* Guard keyFunction against null items, add revert to null identity test