[chap3] maxRating() made more robust and component class comment updated

Chapter 3 changes:
- `maxRating(value)` made more robust; it can now handle when
`value.isEmpty()`, which can happen when `max-rating=“{{…}}”` using the mustache directive.
- Updated class comment: we do not use the `@NgComponent` `map` field anymore.

The changes listed above as well as a Chapter 3 changes made in a previous commit (e83e24388710f7c2b74d651d0d0652eb5766ba5c) are propagated to subsequent chapters too.